Try e-gunparts.com
gun parts corp has them on their website.
Your harrington and richardson model 900 was made between the years 1962-1973.
Blue Book of Gun Values
You will have to measure the inside diameter of the barrel to find out if it is not marked.
Have you tried a web search? Local Library? Do a web search on Harrington and Richardson Books and you will find some sources.
Try the following link. I saw it there. www.gungarage.com/gunmanuals.htm
Does not exist. You might find a magazine at egunparts.
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.
The H&R topper model 490 JR was made from 1961-1983.I hope that you find this informative.
If you took it apart without an owner's manual and now can't get it back together, find a gunsmith.
Cornell Publications
Try e-gunparts.com