Not always the easiest thing to do. The manufacturers still stock many. John S. Craig sells photo-related publications of all types, including both used instruction books and high-quality reprints.
Manuals2go.com offers PDF copies of hundreds of original equipment manuals. Most sell for $6.99 as a download or can be ordered as CD-ROMS or printed out. Mark also offers repair manuals on many products.
Where I know of a special department set up to field requests for instruction books, I've listed it below. Otherwise you may want to check our vendor listing for the central office.
Canon To purchase an operator's manual for any Canon camera or camcorder, please call Canon's Consumer Information Center at 1-800-828-4040.
Online instruction books for Chinon, Ricoh, Sears, Konica, Pentax and many other cameras. This is a private individual's labor of love - he does it for free! About 350 models at http://www.orphancameras.com
also advises me of these additional on-line resources:
Fuji amateur cameras: Owner's manuals for amateur cameras (35mm and 24mm Advanced Photo System) can be ordered by calling (800) 800-3854.
Minolta is unique (at this time) in offering free instruction manuals over the internet. To download a book in Adobe Acrobat PDF format, click here and follow the links.
Nikon Customer Information: 1-800-NIKON-US (1-800-645-6687). You can order on line from http://www.nikonmall.com
Pentax - If you would like to order an instructional manual for a PENTAX Product, please call their Literature Department at (800) 729-1419 ext. 307. They are able to provide (via US Mail) manuals for all current, and most non-current products. There is a $4 charge for each instructional manual to cover shipping and handling. This service is available for United States residents only.