I get all my maintenance filters, seals, and fluids locally at Walmart, NAPA, Carquest, and the local bearing merchant. Cheaper than the "official" prices, excellent quality, always in stock, and I can see what I'm getting before I leave. In over 80,000 miles of riding Bricks I've never been disappointed.
MaxBMW, Motobins, Euro Moto Electric and eBay have been very good to me for the stuff I can't source locally. As far as BBY, I've found it's not too hard to get better deals than they offer on the stuff they actually have in stock. Of course, your mileage may vary.