I agree with astroAPhi. A local Alumnae Panhellenic might also be a good resource.
If you do find yourself having to approach alums directly, though, I'd stick with people you already know, and I'd preface it with something like "I'm going to rush at Random U this fall. They have chapters of ABC, DE, and FGH. I'm keeping my options open, of course, but I know you're an ABC and I was wondering if you could tell me..." This makes it clear that you're not interested in only ABC, you're just picking her brain a bit because she's a friend/relative/teacher/etc. And it could get you a recommendation
33girl, good advice as always. Even in the relative anonymity of the internet, showing interest in only one or two GLOs can easily get back to the school you're rushing at, and other chapters may decide "she really wants to be an ABC, so let's cut her and instead invite someone who actually might want to be an FGH."
