I've found that searching "poodle" with the size "large" tends to get better results. You don't get the toy poodles, etc. Or you can try searching with "standard poodle", that's worked for me too. That's all I've figured out though.
Sure wish they made a "labradoodle" and "goldendoodle" category. I think Petfinder added "cockapoo" recently, so maybe if people e-mail them they'll consider adding doodles to the list. The people who run the site are really nice.
Have you heard of PetHarbor.com? A bunch of shelters that don't post on Petfinder post their pets there. I've seen a few doodles in the past. Its kind of a depressing site though, since some of the high kill shelters (NYC, LA) post on that site.
I actually think that it would be useful to be able to select any two breeds that you want when you are looking for a mix. That way they don't have to add new categories and you can pick any two breeds that you want. I suggested this to them and they said that they are working on adding that option! So - be on the lookout!!