I found a good source w/ excellent pricing for front hub bearing assemblies or rear wheel bearings:
http://www.hubbearing.com/They show 1991 318 front hub bearing assemblies, manufactured by TIMKEN (excellent bearing manufacturer) for only $90.49 each. The TIMKEN rear wheel bearings are listed @ $63.99 each.
There is FREE SHIPPING on all orders over $50.00!
Based on my research so far at some of the well known online BMW parts suppliers/vendors and even the well known local & country wide auto parts stores, the price for just the front hub bearing assembllies from hubbearing.com is the best price I've found... Some of the online BMW parts vendors/suppliers and the local stores want in excess of $115-$130+ each...
I've personally used TIMKEN bearings in multiple vehicle applications over the years, everything from front/rear wheel bearings, rear gear/diff assemblies, bearing seals and clutch throw out bearings and never ever had a failure or issue w/ TIMKEN bearings/seals. TIMKEN is a very well known bearing/seal manufacturer and a supplier to many Automotive manufacturers.
Hopefully this will help someone save some $$$ if needing to replace those parts.