It says like this...

This spell creates a magical link between a Large or larger inanimate plant within range and another plant, at any distance, on the same plane of existence. You must have seen or touched the destination plant at least once before. 

Then what if I've seen that plant using magic or by other means like scrying ball, or in dreams?

(providing that the plant is there for real) Is it still OK if I haven't seen it with my REAL eyes?

Pretty much yes. I'd say through scrying for sure, but I'm a bit on the fence about dreams. If I was DM for that I'd have to consider it on a situational basis. If the dream was say, some kind of divine sending specifically meant to show you something I'd probably allow it. 

This is the same as the question "Can I wild shape into beasts that I've only ever seen when I conjured them?" or "is it easier to scry on someone if a friend shows me what they look like with magic?" and similar types of questions, and I'd really agree that this is DM's discretion.

Personally, I think that scrying is reasonable, but dreams are stretching it. I (as a DM) would rule that scrying on a tree lets you pass through it, but not dreaming of it.

Ultimately, though, it's up to your DM.

I think a dream or any other spell-like imposed dream is as credible as a scry.

If you're a Circle of Dreams druid I'd allow the dream bit. It is your stock in trade after all.

Scrying "Instead of targeting a creature, you can choose a location you have seen before as the target of this spell. When you do, the sensor appears at that location and doesn't move."

The Critical Role way of transport Scry then Transport via Plants, by RAW, is incorrect and can break the game. But if you and the DM are ok with it then go for it.

Quote from MrBangala >>

Scrying "Instead of targeting a creature, you can choose a location you have seen before as the target of this spell. When you do, the sensor appears at that location and doesn't move."

 But if you are scrying on a creature, and the plant is within the viewing radius of the scrying spell, then it would be allowed.

So you could potentially persuade an animal to fly to a location, scry on the animal, see the tree, and then use transport via plants to go there. a bit of a chain to work with but I'd reward that with success.

Do the plants need to be planted on the ground? I ask because I got an idea to give me range. I want to give people plants so that I can have connections almost anywhere I've been. Also, do they need to be alive?

The plant needs to be large, so as long as you can keep a large plant alive off the ground. Sure.

Yes it need to be alive.

It is also worth considering that the Plant can be subject to Enlarge/Reduce, so you could keep a plant that is medium sized to enlarge to large size to use for the casting.

What will also be worth considering is when does a plant cease to be a plant - when it is cut? when it is no longer alive? If you keep a medium sunflower in a vase, does that count if you enlarge it? If you consider a branch to count, then does a staff?

If a rune knight drapes themselves in creeper (thus wearing it) and then makes themselves and anything they are wearing large using Giant Might, can the now-large creeper be used for Transport Via Plants?