Sometimes I find searching can be a bit hit and miss really...ah well.
Anyway, I fitted seats from a MK2 Astra GTE (any of the upper end Astra MK2s will do, they are fitted with Recaros...and yes, made by Recaro). They are seriously cmfy (I have done many thousand miles in them) and offer very good support.
I strenghtened the seatbase first...mine seemed very flimsy to be bolting stuff to it, so made it a bit more rigid by rivetng some panels in made a huge difference. Middle section for to access left as screws obviously..
I then made up some brackets for the seats (bolted to the origina seat brackets). Used rivet nuts for mounting...to me this was the best way, as it meant when the seats do need to come out, it will be a one person job...rather than faffing about with nuts and bolts in difficult to get places...