You're pretty much better off by assembling your own set of strings when you go that low. My band used to tune down to A, and I had to get my guitar set up specifically for that tuning. I don't remember all the measurements of the strings I used, but it was something like a .60mm for the Low E, and .12mm for the High E. The G string ended up being wound, but that is not totally unheard of.
The other guy in the band bought .11 gague strings, threw away the .11mm E, and bought something like a .58 or .60mm string for the low E.
Hope that helps.
edit: My band, not my bad.