Home Depot has a better price ($3.49) here than Ace, and tons of stock. Ace, OTOH, had a couple of Mag Charger bulbs too.
RS and Sears have 3 cell xenon PR bulbs, and Wal-Mart has the 800mA 2 cell ones, but 4 cell are hard to find (other than MagNum Star Xenon).
Sears' new aluminum tool lights are using the 4 cell xenons, so I imagine they'll stock replacements one day...or at least I assume they're xenon, they have that barrel shaped envelope.
Sears also has "5-cell" xenon bulbs, the 6V 975mA ones for the Ultra-Beam SLA rechargeable light. The ones I got in the light and the spares are normal bulb shape, but some of the lanterns on the shelf had the barrel envelope.