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.
try bob`s gun parts. they carry parts for lots of old guns
Numrich gun parts
you can find parts for your Stevens at bob`s gun parts .com
We have parts for that model. firstname.lastname@example.org
On line auctions, gun shops, gun shows, pawn shops want ad, for sale ad.
Try Nurmich Arms
Check with a gunsmith, or find the model number of your rifle, and check on line parts supply houses, like gunpartscorp.com
Numrich Arms is a good source for gun parts.
Go to Numrich Gun Parts Corp.
To find out how old a Winchester rifle is one would need to take the rifle to an appraiser or gun shop. The gun shop will be able to look up the gun by the serial number.
i am also looking for parts so if you find any let me know.... email@example.com
You will have to contact SIG to find out.
We MAY be able to find answer for you, if you can find the MAKE of the rifle for us.
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.
In the age of the Internet a simple search with terms like "Ross Rifle" or "Ross Rifle parts" will turn up various dealers. Including your address, if you are into bricks and mortar stores to shop at will find local dealers.
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.
Do a search for High Standard A1041 rifle parts. Numrich gun parts has a good selection of parts for this rifle.
You can find a firing pin for the ranger at Numerick Gun Parts Corporation.
It is the same as a Winchester model 141 and parts are available from Numrich gun parts
That model is obsolete, but we have a good selection of parts. firstname.lastname@example.org
Try GunPartsCorp....can't find exact email address at this time.....but they have lots of parts for old guns. Good luck.