Where is the best place to get a white mouse online?
You'd probably have the best luck at a pet store, unless you happen to have a friend that breeds them (or a local breeder). However, if you go to the pet store, make sure they know you want a pet and not food for another :) While feeder mice are cute, cuddly, white, and everything else you are looking for - they're specifically bred for food and sometimes not under the best of conditions.
Domesticated mice tend to be a bit bigger, live longer and have been handled properly since breeding - making them a bit more naturally curious around humans and less likely to bite if frightened.
I recommend buying them in pairs, and getting a habitat large enough for the two of them to run around. Wheels, tubes and other accessories help to keep them active which leads to longer life.
Most pet stores sell white mice for both pet and reptile feeder purposes. Choose one for pet purposes, as feeder mice are intended for food and are potentially genetically inferior, possibly due to inbreeding.
That having been said, using a rescued feeder mouse with a potentially short life-span could be a good training tool to familiarize oneself with mouse pet care. Unfortunately for the feeder mouse, I suspect the monetary investment difference between feeder-intended and pet-intended mice is negligible; therefore, you're better off selecting a pet-intended mouse.