They do get loans from banks but, just like everyone else (irrespective of whether they're getting a loan from a bank or another source), they can sometimes be required to provide a guarantor, so that the lender can be sure of getting their money back.
The actual source of Bojo's loan hasn't been made public. Richard Sharp was approached as a possible guarantor, not as a lender, and he then introduced Bojo to someone else who could become the guarantor.