S3 seats are just the same as S-line seats, so no need to specifically look for S3 only seats.
You'll be able to find a set of half leather S-Line seats on eBay fairly easily. If you're lucky you'll find a set with electric lumbar. They also have an adjustable thigh rest which may take the pressure off your back and stop the pain from happening.
Audi seats have always been rubbish, other than when they employed Recaro to do the seats like in the 8L S3, sadly there arent many options in the 8V as even the Supersport seats are pretty much identical to the S-line seats other than some additional bolstering on the rear section.