Bastien as Winston mentioned there is a company called
Pet Card please have a look at the website, they offer two options either a loan or an emergency use credit care for vet care.
A full panel blood and urine tests is a good place to start and would help the vets to understand what treatments are needed right away. These two tests together should cost around $250 or less.
Ask to speak to the office manager of your regular vet, they should be able to work out a payment plan option for a regular client that has a pet in need.