This is what credit cards are good for, and I don't know any vets that don't take them. Pay the balance a month later and it costs you nothing for the "loan". Large animal vets, at least where I live, are much more lenient. I have never, ever had to pay on receipt of services. They bill me, and even then I get a month to have it paid before interest is added. In fact, my vet gives $5 off if you pay within a certain time frame (20 days?). But then, I can get a routine visit where the mobile fee (farm call) is less than I would pay to take my dog to the small animal vet and have him looked at for a couple of minutes!
That said, if I had been using a vet for 15 years and they wouldn't give me a week to pony up, that would likely be enough to have me vet shopping. If loyalty buys you nothing, then why be loyal to them??