A lot of reputable places online do have prices that beat many store prices. However, a lot of contractors are wary of people buying them and usually won't put them in if you have pre-purchased it from some iffy place online. It kind of seems like taking your car to the garage and having the parts. They don't know if they're defective or where they really came from, so they can't honor the warranty and usually won't even put the parts on for you.
If the water pipe valves on top won't turn, I'd get them replaced. If you try to force them and they break, you're going to be without any water unless there are cutoffs that work further up the line without turning off water to vital places in the house. Otherwise, you'll be stuck with turning off the water out by the street while those valves are being replaced.