Last time I was looking I checked several places:
http://findcollegerentals.com/
online message/ad boards for local colleges
local property/realty companies' websites
I can remember under which one I found my current rental, but not many advertised being pet friendly. I would always have to ask and offered to pay a large pet deposit and provide proof of rabies certificate and spay/neuter. Our current landlords were against pets, but I was very transparent with them and they changed their mind. They even let us add a second dog (and I have three cats).

http://vankelderdogs.dutchbingo.net
http://www.schhmi.com