Hi there.
As a former collector of flight helmets I can shed a little lite on this for you and hopefully help you out a little bit. About the only place that you can get helmets that I have experience with is E-bay, That is where I bought mine. (before getting married and buying a house, now I don't have the cash to throw at it) Cost depends on what your looking for, some of the Russian helmets can be pretty reasonable cost wise. If your looking for a good quality US helmet and mask set up, you can expect to spend between $300.00 and sometimes up to $700.00 dollars. They can even go for more than that depending on squadron decorations and such. You also have to be pretty careful do some research, there are folks out there who will make a motorcycle helmet look like the real deal when its total garbage.
Firefly.