I went with a 135/80-18 sized spare tire (Pirelli PSpare) from Tirerack.com
With a 26.5" diameter, this is a nearly perfect match for a rear 275/35-19 tire (calculates to 26.6"). I specifically chose this to minimize a L/R diameter difference on the rear differential. I figured that matching the diameter was less critical on the front.
Haven't had to use it yet, but I did install the spare wheel on a front corner when I removed a front wheel to inspect it more closely.
2011 e92 M3: AW/blk zcp, dct
2008 Civic Si FG2 ASM/blk
2005 e46 M3: SG/blk zcp, 6MT (sold)