Many of the forklift engines repair manuals are available for a small fee from the forklift manufacturer. That is unless the lift truck you need the manual for is really old. If this is the case, they may not have anymore copies left of the manual to sell you.
Go to the forklift manufacturers website and see if there is anything there about manuals and how to get one. If you are unsuccessful, try old bookstores who specialize in repai manuals. You may also try to hunt for forums and clubs who specialise in old machinery. Try ebay and other auction sites. Good luck.