17in to 18in rims 4 bolts

E-Bay would be a good starting point. However, be wary that you may have to roll your fenders to accommodate those rims. 18's on an E30 body are a tight fit. Also, make sure you are buying rims with the correct offset. Hope this helps.

you can fit up to a 19" wheel but then you couldnt lower the car....and a wheel that big just looks funny. anyways e30s are all about the proper offset. you can run pretty much any honda/vw/acura wheel in a 4x100 bolt pattern. the issue comes with proper offset. you need about a 25 offset wheel to fit over the hubs and not poke out past the fenders. so if you got a standard honda wheel in a 4x100 with a 45 offset you would need a 20mm spacer and longer wheel bolts. i recommend going to wheel studs as it makes putting spacers then hanging the wheel easier. i also strongly recommend H&R Trak Spacers as they are aluminum and CNC'd from solid aluminum chunks which makes for a stronger part. hope that helps. heres a pic of my old e30 on VW offset wheels, with studs and H&R spacers

You could also do the M3 5-lug conversion so you can use any E36 bolt pattern wheel. You can also get the benefits of larger M3 brakes when you do this. I'd also suggest looking at The Tire Rack ( http://www.tirerack.com ), they have a virtual wheel program so you can see the wheels on your car before you buy. I'd suggest 17's though, 18's introduce a lot of unsprung weight to the equation and will perform worse than a lighter wheel.