If it's small and must be focusable, you are highly restricted. This basically limits you to Minimag-type lightbulbs. If you drop the requirement for focusability, then you can step to a Lambda Illuminator (don't know if he is still making these but my LI was a great EDC for a long time for me) or the ARC LS + 2AA tailpack (without a doubt the most well made light in this class). Beyond that, you're into limited distribution, one-offs and contract mods (read: expensive). You'll have to decide just how much trouble/effort you're willing to go through.
Perhaps if you told us a little about your application. Is it just for general EDC? If this is the case, I'd just bite the bullet and buy a Surefire L4. This light is the all time finest EDC light. Small, bright, rugged and using Pila lithium ion batteries, very economical and ecologically friendly to operate.
Wilkey