Try local gun shops, the yellow and white pages.
You can find a 22 mag cylinder for a 22 mag revolver mod 21S MIW Germany on eBay. You may also be able to find one through antique sales and antique gun collectors.
It would be best to take it to a gun shop to have it Identified.
Try the yellow pages and look for auction houses, antique stores, gun stores.
Gunsmiths, gun collectors, gun-oriented internet forums, you might find someone at the gun show who'll know something about it.
A good place to meet some online is antiqueguns.com. They are always willing to give an answer and some of them wrote "the book" in their fields. If you want to meet collectors in person, watch your local media for ads for upcoming gunshows.
No
An old gun that has been around for a while and was widely popular and is now presently not in production and is a rarity to find around anymore.
You can often find a Winchester Centennial 66 at a gun show or in the hands of collectors. Due to their age, they are not likely to be found at gun shops.
No
Library, gun shop, gun shows.
Blue Book of Gun Values, The Parker Gun Collectors Association
Value of ANY gun will be based on condition- and in the case of an antique, originality. DO NOT clean up an antique- what you call dirt, collectors call patina. There is no way to give a value without a hands-on assessment by a person that knows antique British pistols. It may be $50, or $500.