First, that level of ketone is not something to get frantic about. Make sure Mimi is eating, get as much liquid as you can into her, and keep testing for ketones so you know what's going on.
Second, no medical or veterinary professional will hand you a prescription without an office visit unless it's a prescription renewal and even then, they are supposed to see you annually versus just writing the Rx. It's their license. And while a vet visit may seem like it's an unnecessary expense, what happens if you ask for a medication you think will be fine and it harms your cat? You may not blame and/or sue the vet but many other people would because the "vet should have known." Frankly, I tend to not be a big fan of people advising that you give your cat a prescription medication unless it is abundantly clear what the problem is and you've given your cat that medication in the past.