Past customers is definitely the place to start.
As for how much to charge you have to look at how much it's worth to them, you want to give good value otherwise in the long run neither of you wins.
Lets say they only have 100 customers on file and you get a 5% response (because you
will have an offer in there, right

). How much is 5 sales worth to them? Is it going to be enough for you to charge what you want?
Personally I'd put together a whole "marketing kit" for a vet consisting of a whole years worth of newsletters, ads, website, everything you can get your hands on. You don't have to actually make it, just the concept.
Get involved at the very beginning of the marketing cycle, by driving more customers to them in the first place you'll make much better use of the newsletter and other materials later on.
If you can sell just 1 vet on this you'll be a lot better off than trying to write 1 newsletter at a time for 5 different vets where you have no control on the rest of the system.