Usually, at local pet shops, you can buy individual frozen feeders. At the big chain pet stores, they sell commercially packaged feeders. The weight printed on the box is just an average weight. So, if you buy a box of two small rats, the Net Wt on the box is 100 grams, so each of the rats will be somewhere around 50 grams each +/- a few grams.
My local pet store gets their frozen feeders from a rat breeder, so they basically have a bunch of comparably sized rats in Zip-lock bags. When I buy those, I take a scale with me to the pet store to weigh the feeders, so I can make sure that I'm getting the appropriate sizes.