If you haven't already, call customer service at S&W. They still have some parts for the 3rd Gen guns. If not, Midway, Brownells, and Numrich also have some parts.
Finally Ebay often has parts listed.
If you are asking of the 5906 and 6906 parts are interchangeable, the answer is probably. I just did a swap between my early 6906 and 5906 and the parts interchanged.