Just about any store which carries fish will have ghost shrimp (or grass shrimp, which also work). That is because these shrimp are often simply used as feeder food, and so are quite cheap and common. Most of my LFS sell them for 5-10 shrimp per dollar, so, quite cheap.
When you do go to get them, be sure to look at the tank with the shrimp and study them for a few minutes. Some pet stores do not take good care of their shrimp populations because many people buy them just for fish food, and so think 'who care? They're just going to get eaten." Some of these stores will have very sickly looking shrimp, if that is the case, and you have another pet store available, I recommend going to the other store to see if conditions are better there - it's no good putting sick shrimp in your betta's tank, I can assure you he really won't appreciate any diseases his new company brings with them!