I know the celica front seats from the 7th gen can fit into our 6gc with mods to the seat rail and swapping out the cushions, but for the rear are there any alternatives to the rear seats from other cars or aftermarkets?

I think the rear seats are easily replaced with the ones from 7gen... Use the search phrase "site:www.6gc.net 7gen rear seats" in Google and check the topics, you'll find everything you need to know about it, looks like a perfect fit.

I doubt any other rear seats would fit, not even sure if I ever heard about AEM rear seats.

yup, 7th gen rear seats fit right in.

Rear 7th gen seats fit. I have them in my car. The only issue with them is that the bottom cushion doesn't fit perfectly so there's a small gap.

I have gotten the 7gen rear seats in, it doesn't fit well for the 94' and there is a half inch gap, and the back lumbar seats don't fit well with my alignment of holes, just curious if here were anything else out there, not looking into reupholstered - last resort.