I'm playing a Nightingale build with some Illusion but I can't seem to find anyone who sells higher level spells. I have no interest in joining the College on this character either.

The College is your only option.

Or in

Damn, i was really hoping to avoid the College but oh well. I'll give Apocrypha a try.

Yeah, from what I know, you will need to join the College for high level spells. You don't have to follow the quest line though, just join them and you can buy all you want.

I'm dealing with this right now. I've been to Apocrypha but nothing good showed up. For example, my PC qualifies for Adept-level restoration spells but nothing that high has ever showed up in loot.

I didn't want to buy spells at all, but that's impractical so I bought some basics to get started. Now, at level 53, I haven't improved the spell selection through loot so it looks like there's not much choice. I did join the college just to get to the Midden 'cause I couldn't find the way inside from the beach but I haven't bought anything from the staff.

I also hoped my spouse's store would start stocking spell tomes I can use ( a kind of compromise to buying from the college) but that hasn't worked either.