I have recently found two additional companies that have been pretty useful.
For a desk I am building, I a mirror / glass trim has been specified as a trim made by Extrude-A-Trim. They have several unique shapes, and no minimum quantities. However they are located in Canada, so their shipping charges may be more expensive.
I also use a variety of stand-offs, which I usually purchase through Outwater. However, Metomic has a few cheaper alternatives. Of course if you need to go high end, Gyford is the way to go.
I think that one of the interesting things about these types of hardware is that I would have used them more in theatre if I had known they exsisted. Both in design and in technical direction, there is a pull between using "safe" hardware - its known, available, and economical & that "perfect" piece of hardware that may be commerically available, or may need to be custom built, or it may be available but expensive.