If you have tried your bank and have been declined a used car loan on the basis of your bad credit rating then, you have little choice but to forget any bank finance from any bank. It simply won't happen. Your car loan whether for a new car or for a used car will have to be sourced from a non bank lender.
Many finance companies out there now specialise in this market because the bank qualification process has tightened up since the credit crunch and that means more people like you and less lenders that will assist them. If you get your head around paying more for finance than you anticipated, then there are plenty of finance sources that will finance bad credit clients with car finance.