Snyder's is great, but also consider calling Premium Liquor on 9th, by the Convention Center. Anytime I've gone there to ask for something, I find it the next time I stop in (without me asking for them to order it). They have a stellar selection.
I was just in Schneider's of Capitol Hill (not Snyder's) on Saturday to buy some gin. They usually have Old Tom, but they were out. They do have some other nice 'artisanal' gins, but I cannot recall the name of the one we were looking at. They can always order stuff though.