I'm trying to create a portal through which my party will chase an evil magic user. They will run into the portal and come out in a city about 150 miles away. I assumed this would be an easy enough thing to do in D&D but when I looked for items and spells, I didn't find what I was looking for. Can you guys help me find a solution that won't result in them obtaining some kind of priceless artifact in the process?

I was thinking maybe the wizard has casts a Gate spell that leads to something like a Rope Trick or some other-planar space which leads back to the city (I don't think Rope Trick does this though, and it wouldn't last long enough)? I just don't see a spell that opens a portal 150 miles away. I mean I could just say "it is so," but there's at least  one player in the group that will know the lore well enough to question it and I'd prefer to be able to back it up if they choose to investigate it as a matter of arcana. 

Thanks :)

There are all sorts of abilities open to NPCs and Monsters which aren't available to Characters. Having NPCs/Monsters with access to spells or traits which the Characters aren't familiar isn't a big deal. "It is so" is a perfectly viable thing for the GM to say, and it doesn't require a whole lot of explanation.

However, Teleportation Circle has the proviso: You can create a permanent teleportation circle by casting this spell in the same location every day for one year. 

It also reaches anywhere there is a corresponding circle on the same plane of existence, and it stays open for 1 round/6 seconds.

If the Party is fast enough, they could easily follow the evil Wizard through the circle.

You could also manage to slip the Party a means of keeping a circle open longer, if you feel that's needed. Something consumable shouldn't unbalance the game overly.

Keep in mind that, as the DM, you're not limited exclusively to the same rules that apply to characters. If you want to have a portal that just stays open at all times, you can just say that one such portal exists in the location the players get to. You can come up with any kind of story for why it exists, even if it doesn't perfectly fit the spells that are available to characters.

You can just do it and if they get a high enough Arcana check you can just say “It’s nothing quite like anything you have ever seen before.” 

Okay that's great, thanks all. The plan is to have the wizard be connected to a group of wizards so it's easy to imagine that they've come up with something the party has never heard of. Also this is a secret portal the magic user has been using for a long time, so that satisfies the "casting in the same location every day for a year" possibility as well, should they investigate it. 

That'll work! Just needed some fodder for an explanation (or excuse). Thanks again!

See?  This is one of those things I expected in 5E with Faerun as the default setting.  There are lots of Portals in Faerun in the books.  So, this falls under handwaving territory to me.  Just create it.  If one of your players thinks they want to recreate this, say that if they cast the spell in the same location every day for a year that it will become permanent (there is plenty of that available in the spells... and who knows, maybe some of the longer lived races did this in the past).  They aren't going to be out in the open... they should be tucked away because this kind of magic is very powerful and would have required great expense in time and effort on the part of the casters of old.  But absolutely... add in Portals.  They can make things interesting.  Most likely it is going to be how much the players are going to use it against you though... instead of the other way around.  And maybe if they rely on it too much, ever now and then it glitches and sends people somewhere else... which is why no one ever uses it any more and it fell into disuse and then was forgotten...

Quote from Grayfax >>

.  But absolutely... add in Portals.  They can make things interesting.