Does not exist. You might find a magazine at egunparts.
Your harrington and richardson model 900 was made between the years 1962-1973.
Gun shops, guns shows, pawn shops, want ads, e-gunparts.com
Try Gun Parts Corp
Blue Book of Gun Values
Have you tried a web search? Local Library? Do a web search on Harrington and Richardson Books and you will find some sources.
You will have to measure the inside diameter of the barrel to find out if it is not marked.
Try e-gunparts.com
Try the following link. I saw it there. www.gungarage.com/gunmanuals.htm
gun parts corp has them on their website.
The Harrington & Richardson 349 Gamester shotgun does not use a clip. It has a tube type magazine that is a pemanent part of the gun, loaded by sliding shells in from the bittom of the gun.
this shotgun was made by Harrington & Richardson and is there model 120.www.e-gunparts.com sells some parts for this gun. searching under Harrington & Richardson model 120 will yield more results for you. good luck.