I've tried the college of winterhold and some other towns, but all I find is alchemists and such.

Level up your magic skills and then the college of winterhold should update the spells they have according to your level. That's how I got my invisibility spell

All the court wizards sell them. They're usually in a side room in the keep, in the bigger holds.

Riverwood Trader has a few

Falion in Morthal

Some dungeons have them. I like the transmute spell, You can make some nice money turning iron ore into gold.

the best selection of spell tomes is from the different teachers in the college at winterhold... and they add more as you level all the way up to the radiant quest at level 90 to give you uber funness... and then go back to them when you complete it and they'll have other versions or other spells the same level.

random spell tomes spawn inside of dungeons, to include waterbreathing and telekenesis at Labrynthian in different games for me.

Most Court Wizards also sell spell tombs. But yes, the more you level your magic, the more books/tombs will appear in the available merchant section. Mages College is definitely the best one stop shop, as others pointed out.

Like everyone else says above.... The court wizards of major holds.

I have always purchased spell books from the wizard in dragonsreach myself as it seems he has the entire spell book inventory.

Read on the wiki that there are two people (college and riften) that are supposed to sell the Telekenesis tome, but neither one is offering it in their inventory, nor any of the other court wizards, is this a bug or is the wiki wrong?

It's an Adept spell. IIRC, wizards will only have them in stock if your Alteration skill if 40 or higher. Maybe you need to level them up?