We have pet insurance on our gang. However, it has to go on credit card first then when they process the claims we repay back the card.
I would ask to speak with the office manager then go to the vet. I know where we were yesterday (specialty vet) they (office girls) were talking about someone who had a balance. I know they can if they want work with people. It would be curl to refuse treatment just because someone cant pay. They dont do it for humans it should be the same consideration for animals.
Other suggestion is getting personal loan.