The Munson Feeding Plan is a great resource - there's load of threads on here about it but in general it tells you how often and how much to feed your snake based on your snakes weight. Many of us that use are more conservative as it's considered a very aggressive plan but you'll find more details about it on a dedicated thread about it. (Buy a digital scale, very helpful!)
I'd keep her on the size she's currently eating with the breeder, just to play it safe and once you get her you'll be able to better tell what to feed her.
I'm not sure about ordering food online as I live in Scotland, but I buy four mice at a time since I don't need to worry about delivery prices. I think people buy them in bulk but someone else will be more helpful here.
Hope that helps you a bit, good luck with your new snake when you get her!!
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