Personally, I would not use the HD ones....I have seen them crack many a tank....
There is a C.R Lawrence company on the south shore somewhere...they usually have diamond bits in stock...but you may need a wholesale account to purchase from them (not really sure) but they are the only place I have found that has them in stock when I need them in a pinch
What size do you need??? I have some extra ones from glass-holes.com that I do not use cause they have a short shank, and our drill press requires long shank bits...
(sorry if I od not respond till later tonight, as I am about to leave the shop to go on the road for service for the next 6-10 hours...)