Depends on the area, mostly. I've never seen one in a pet store, I bought the original siamese girl and her sister from a friend that also breeds rats. That was at the Scott Smith expo in Wheaton, IL, and I'm sure I could have found at least one other rat breeder there who had some. They're a recessive trait though, so they tend to get drowned out in large breeding operations. I'd say they're about as rare as hairless - not difficult to find if you're specifically looking for them, but you're unlikely to see them in regular shops.
If yours is nasty then by all means feed her off if thats what you want. No use holding something back to breed if its got a bad temperament. Mine are a bit skittish due to a lack of handling but I haven't been bit by any rat I've bred (excluding one crotchety old man rat, but he was over 2 years old and I'd been handling him longer than he liked, lol, so that was my fault)