Well, I know you seem to be more interested in cheaper replacements, but if you wanted to invest in some really nice seats, check out Cobra -
www.cobraseats.com. Under the "Street" section, the Le Mans seat is pretty similar to the original E30 sport seats. They are all custom made for your application, and they will cover them in whatever you want. They are pretty pricey, but man, they are cool as ALL hell!
Alternatively, you can likely get replacement parts for your seats (new springs and whatnot) from the BMW dealer. I actually recovered the driver's side seat bottom in my car myself with a new leatherette seat cover from my local BMW dealer. They looked up the part number and ordered the new cover from Germany for me. It was pretty reasonable at about $40 for the seat cover, and BMW even overnighted it via FedEx from Germany to the dealer (which I did not have to pay for!). If you think you'd be comfortable recovering your seats, I really don't think it'd cost you too much to fix the seats with new parts. You'd be very surprised at the extent of parts that can still easily be obtained from the dealer for these cars.
If you go that route, I can give you some pointers on how to recover the seats if you need help (provided you don't have the sport seats). It's actually pretty easy with the standard non-sport seats (it was a hell of a lot easier than recovering the seats in my Camaro). You just need one specialty tool - a set of angled hog-ring pliers and a handful of hogrings. Took me about an hour to disassemble the seat, recover the seat bottom and put the seat back together.