Dave: Parts can still be obtained from a number of sources. Numrich comes to mind, the other is that fellow in Arizona (Dave's?). Look for a long sting of posts over on Shotgunworld (shotgun ID and value forum, currently page 3) on the 520. For years...(& largely because of some ongoing nefarious activities in Chicopee Falls) there was simply no cohesive information on these John M. Browning designed guns, making them, sadly, JMB's only red-headed step-children. Good information on them does now exist.
Also on the Shotgunworld webpage, look for Okiemarine's fairly definitive 520 Identification Guide in the Shotgun ID and Value archive section. Also, here:
http://www.gunvaluesboard.com/stevens-520-and-520a-identification-guide-2617.html#comment-7683
