I looked into something similar before developing in another niche....
Seemed to me you have to contact individual lenders..... I typed in "loan affiliate program" "finance affiliate program" etc
This brings up lots of options, I spoke to a few and all said the site would have to be and running and generating some traffic before sign up which is standard...
awin have a selection of finance companies to choose from..... I'm signed up with them and have seen some floating around, though I'm in the UK so not sure on the options available in your neck of the woods....
You can also check out the other major affiliate networks and see what they have on their books....