Brownell's carries quite a few on their website. Numrich Gun Parts carries a lot of older and obsolete firearms on theirs. i have used both for many of my firearms. another source is the manufacteror themselves. most of the firearms i own that have owners manuals have exploded views of the firearm with a parts list and sometime even the part numbers. BTW, welcome to the forum and stop over in the introductions section and say hello! again welcome to the forum.
Welcome! As said, Brownells, gunpartscorp, and the maker's websites. For older Mossbergs. damguy's Mossberg site, and steve's pages. A lot of milsurp here- http://www.surplusrifle.com/
I use yahoo, google will work to. Type the name of the gun and exploded view. That will give you a diagram of all of the parts. Unfortunately not all guns are there. My baby eagle for example isn't but the eaa witness is almost identical and the eaa witness is there. As for breaking them down and putting them back together that depends on your skills. I took my baby eagle completely apart and put it back together with no problems. Mostly just remembering where everything goes. The exploded views help but if you have ability you can take them down and put them back together easily.
google by the firearm name + schematic For example: "Mossin Nagant schematic" with or without the quotes. You'll be surprised what you can sometimes come up with. The likely sources will be brownells and gunpartscorp
