I can get just about every size nut and bolt in stainless at my local industrial supply store. The 4 control arm bolts that you quoted at $26 I can get for $3.50! I would do some checking around. The place I go is called Valley Industrial Products (VIP). A guy that works there tells me one of their regular customers buys boxes of stainless bolts and makes up kits for eBay. Those black ones are black oxide coated.
Jim