There is a company that sells a partial kit for building derringers. I believe they are called Excalibur. Here's a link to purchase some of the parts.
http://theflatspot.weebly.com/derringer-45lc410-sideplates--breech-face.html
My parts came to $48 minus shipping. The side plates are top notch and worked perfectly. I turned a couple inserts in 32 S&W and 38 SPL for mine. Lots of fun!