If the bank won't loan the money, I'd seek out a mortgage broker who may have access to private investors who can make unique arrangements that bank structured loans can't or won't offer.
FHA only does double wide manufactured homes and they have to be attached properly to their standards.
You can only get a loan on a single wide when you purchase it from the dealer ship. Few if any local banks will handle them. As Jefferson said Seller financing is the only way to go.
I am sorry that we can not give you a good or easy answer.
I agree with Jefferson, you might consider if the owner will sell via land contract.