Auralex is your best bet. Google 'em.
Fair warning, though, Chris... you are in search of the Holy Grail. Short of building sound-smart walls within your space (thereby decreasing your space), there really isn't much that can be done to retrofit a space for good sound.
One option I have seen work - and that NOBODY seems to think of - is to create a "cone of silence" over your kit. Think of those enormous mosquito nets they put over swanky beds in tropical climates. Now replace the mosquito net with heavy velvet theater curtains. Badda-boom, badda-bing... the Cone of Silence! (Or, more accurately, the Cone of Quieter.)
If done right, the curtains can be raised up to the ceiling when the drum kit is not in use, and create a cool-looking ring of swagged drapery up near the ceiling. Kinda cool aesthetic....
And finally, look in to a "White Noise" generator. Ever wonder why you can't hear the people in the next examination room at the Dr's office? White noise generators in the ceiling. (Although the environment they create feels a little... odd...)
Good luck.
