Bookstore, or Amazon.com. However, for REALLY old guns- 1860s and earlier, look for a copy of the Flayderman's Guide to Antique Firearms.
Basic reference.
Blue Book of Gun Values
BLue Book of Gun Values
There are several. The Blue Book of Gun Values, Gun Trader's Guide, etc.
Blue Book of Gun Values or old Gun Digest's
Blue Book of Gun Values
Check the Blue Book of Gun Values
Blue Book of Guns, proofhouse.com
Library, gun show, gun shop, on line
Blue Book of Gun Values might have an answer.
call: 1-800-877-4867 ask for copy of the BLUE BOOK, 28th edition....for gun values cost $39.95 + S&H
Go to a large book store and buy "Bluebook of Guns." or "Catalogue of firearms"