"Where did Cain find a wife who wasn't his sister?"

He didn't. They were from West Virginia.

(actually, if you want the straight answer...

...by Biblical accounts, Adam and Eve both lived to the ripe old age of 900+ years -- as did most of the people (alas, not Abel) that lived before the flood. As Adam and Eve reproduced at a reasonable rate through much of those 900+ years, the "sisters" from whom Cain had to draw were a small town's worth...

...but also, there is no mention in the account that Cain and Abel were the first-born -- just the first mentioned. As the rest of their siblings were also living to 900+ years of age, and reproducing at a reasonable rate in a world without birth control, Cain likely had great-great nieces his own age (as if THAT matters) from which to choose -- especially when the age of maturity probably still allowed for teen-age girls to bear children -- and that's likely to happen in a world without Television or Nintendo -- what else you gonna do but screw?