One of my glass shower doors shattered! How can I get it replaced?
Check the yellow pages for custom glass shops.  They can make or custom order the tempered glass and may have common sizes in stock.  I replaced a large door a few years ago for on the order of $100 IIRC.  Many will do free estimates, and know the market well enough to tell you if you're wasting your time.

It may also be worth looking at some older, pre-Home Depot hardware/home improvement stores in your area as well.  You'd be surprised how static some of these product lines are, and many manufactures do in fact support their products with parts.  

It's probably a long shot, time vs. money and all.  But the help at the box stores typically have no clue, so I wouldn't base much on their opinion.