Does anyone here know of any good resources for a wizard's tower? I have a party of 5 level 6 characters that will be entering one soon and I'm looking for either a premade module that uses one that I can use for inspiration, or ideas for what to fill it with. 

I had one I have used a few times, isn't anything huge or major, Wizard conducting "Teleportation experiments" thinks the party are expected visitors from one of the academies here to see his experiments and sign off on further funding. 

One volunteer is led to a circle on the upper level, stands in the circle, wizard chants an incantation there is smoke, a flash and the character vanishes. Wizard explains that the character can't be returned, that is why he needs more funding. If the players start to get a little bit stressed and then confirm they are not the people with funds Wizard offers to send them to the same place. 

In reality Character fell through a trap door and fell to the basement, as he reached the ground a feather fall enchantment prevents them hurting themselves, but they are trapped, at the bottom of the tower. 

Another option, when they get to the top they trigger a self defence mechanism that shrinks the tower and traps it in a pocket dimension. The players have to try and escape before the tower has completed its shrink, put some items in the tower to help this escape from a tower that is so high up in the air :). 

Many wizard towers "appear" to be simple towers.  But the doors you can open that should lead into empty air lead to all kinds of strange places... from portals to places far away to pocket dimensions to underground labyrinths.  To me, that is the best part of a wizard's tower... it can go anywhere.  And it can all use the same doorway if needed with runes or gems or whatever needed to make it easy for the wizard to be where they need to be and difficult to ascertain where that is by anyone else.  Also, traps... lots of traps in certain sections.

One of the online only expansions for Dragon of IceSpire Peak has a session that occurs in a ruined wizard's tower. If you don't have that, I'd google or look on pinterest - there are a bunch of free battle maps for towers that you can use for inspiration. 

The top levels of Dungeon of the Mad Mage is basically a giant underground wizard tower, unless you mean to use one as an actual dungeon in which case that's probably over-levelled for your party.. 

Here you go:

http://www.trilemma.com/blog/adventures/24%20Sky-Blind%20Spire.pdf

I have a few ideas, feel free to use any you like.

• My first idea requires twin wizards working together. When they arrive at the tower, the wizards are in a bit of a kerfuffle. They accidentally cloned one of the twins, and now there are three of them. The two real brothers don't know which of the other two is the imposter. The imposter, however, acts like a different brother each day. They know the imposter is switching, but they still can't figure out who it is. They need the help of the PCs.
• Another idea is that a wizard is meddling in one of the few things that should never be meddled with: time. He has accidentally reversed the flow of time in his tower, and the rate of reversed time is accelerating. He needs the players to break a magic clock he made that is doing so before he gets unborn. However, a sphinx (they can deal in time, aging and stuff) has found out, and has taken the clock, to stop the wizard from further meddling. They need to retrieve or destroy the clock. However, if they destroy it and don't get back to the wizard in the time it took for them to leave him and destroy it, the time reverses itself, and he forgets he owes them (if money is an incentive).
• My last idea is that the wizard has opened a portal. The thing is, he doesn't know where the portal goes, and is too scared to find out. You can make it go anywhere you want, using any adventure you have planned.

Hope these help!

I designed a wizard's tower for a campaign, but never had the chance to use it yet.

It has three levels of basement.  The lowest two are actually a single "level" double height because of a dragon.  The 'first basement' contains cold storage of barrels of stuff.  The ground floor is actually a guard house.  It is a circular space with a radius of 36ft or something huge.  The outer wall is eight feet thick.  The second level is the "mess" level for the troops.  The third level is the officers and NCO offices.

From the fourth level through to the ninth level are the wizards quarters, but there is a stairwell that leads to the top level of the tower used by sentries.

Level 4 is storage for the wizard and his family/apprentice.

Level 5 is the bedroom level for the wizard's tower.

Level 6 is the living room level for the wizard's tower.

Level 7 is the wizards study.

Level 8 is the wizards alchemy lab.

Level 9 is the wizards experiment lab.

The tenth level is the soldiers observation level used by the soldiers, but the wizard may use it with proper coordination with the king.

Now as I said, the bottom levels are a 36 ft radius circle.  The 'wizard levels' are a 20 ft radius circle, with proper reinforcement below through the lowest levels to support the weight.

The stairwell is a separate 16 foot diameter round tower that runs up touching the wizards tower.  At each level there is a door that runs into the main floor, but the soldiers don't have the 'key' to open these doors.  If they meet the wizard in the stairwell, they are expected to give way to the wizard as he passes by.  The wizard tends to announce himself by casting a light cantrip in the tower that follows him up or down as he goes.  The light simply floats up through the very center of the tower.  Inside the stairwell, on each level, there is a small magical closet tucked in under the stairs which serves as a bathroom.  The unwanted debris is disposed of through magical means.  Outside the 'closet' there is a wash stand.  Nobody has to walk down five or six flights of stairs to use the bathroom.

According to the lore, the tower was originally intended to be built as a military headquarters of the town, and was to be called The Citadel.  Now only the first three levels are called The Citadel, and the remainder is called The Tower, The Stair or The (upper) Observatory.