1.)Shut off the main water line to the house (or to the bathroom, if possible).
2.)Drain the toilet and remove the valve.
3.)Take the valve with me down to Home Depot. Although my preference is a local hardware store as the employees tend to be more knowledgeable.
Unless your commode is a hundred years old, an origional replacement valve should still be available.
Does the old fitting have a nut to turn? If not it is a sweat, you will need a propane blowtorch, flux, emery cloth, and solder. The old fitting can either be removed by heating till the old solder liquifyes, or by using a tubing cutter. The cutter is kind of a pain in tight situations.
Try a local HW store, or an ACE, the chances of finding anyone with knowlege in a big box chain store is very slight.