I ordered a box cap from Mammoth when I forgot to include a particular value in a Mouser order, and it was the exact same size as the Kemets I got from Mouser. Can't remember if the voltage rating or tolerance differed much, but I know it wasn't enough to raise my eyebrows. It worked out fine on the Pork Barrel, which is a pretty tight board.
I agree with Culturejam on ordering caps from Mouser--with the correct lead spacing and voltage plugged in, it's not too hard to get the right thing. Even without enough for a price break (I was only ordering enough for three builds), some of the Kemet box caps came in at $.09 each.