The first place to look would be at your local gunsmith's shop. Next bet would be at a gun show. If you know exactly what you need, you can order some things online.
I'd like to add the following to the above answer.
Gun shows as a source for obsolete parts can be a god send, but you have to be careful as well. Gun shows are known for also being used as a dumping ground for defective parts or of marginal quality (knock offs and counterfeit).
Online sources for parts are:
The Online Gun auctions sites like Gunbroker.com or Ebay, which has reversed a policy decision to not allow firearm parts (The ban came shortly after the Virgina Tech shootings), as always "buyer beware" as those venues are also subject to parts dumping.
Numrich Arms, also known as Gun Parts Corporation (http://www.gunpartscorp.com) is the most well known seller of parts, and has parts for a very large selection of firearms manufacturers, both current and defunct.
Jack First is another well known and established source (http://jackfirstgun.com).
In my past experiences, if those 2 places don't have a part, it's going to be a challenge.
Another possible source, the various firearms related Internet forums. There are dedicated ones for Ruger, Marlin, S&W as well as a vast array of other gun related forums, all usually have classified section where parts are sold.
Regrets- you probably WON'T find them. That rifle was made from 1928-1931. Gunpartscorp is sold out of used parts for that model. You can check with the Mossberg section on the rimfire central forum, or contact Havlin Sales through their website, but the rifle is simply too old, and made for too few years. Good luck in your search.
The Crosman 140 Air Rifle was made from 1954 to 1968 in four different variations. Crosman no longer makes parts for the rifle but you can still get it repaired. The Crosman home page offers a "Find a repair center" button on their first page. From there you can locate a center that has parts for your rifle. See the related link.
Unfortunately your Model 10 Mossberg (one word) rifle was only made from 1934-1936, with the end result that parts will be VERY hard to find. Gunpartscorp has only a few parts for these on their website. havlinsales has many Mossberg .22 parts (and run the Mossberg Collector's Society) Finally, a good gunsmith can make many of the small parts that you rifle may need- but finding parts WILL be difficult. Good luck-
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