www.posthorn.comNot sure if it's the _best_ place, but they have some variety and some of the hardware they sell looks indestructable. They list an ultraclamp near the top of their listing that looks perfect...$33.
I've bought stuff from sam ash and zzounds also.
www.samash.comwww.zzounds.comSam Ash has a variety of uniquely designed clamps but not sure how well they work. I've got one ordered now (the one with a clip on each end) in fact just to check it out to see how rugged and reliable it is for field use.
Sam Ash also has a real nice $10 clamp that attaches to the shaft of a micstand and has a 5/8inch male adapter at the end of a six inch arm, upon which you can attach a T-Bar. Perfect for hitchhiking onto someone elses mic stand at a show.