For the recoil spring, check out Bedair.
For the backstrap, check Brownells. They handle BerettaUSA's parts. If you don't see it on their website (there are schematics for many guns, with the parts linked on that same page), give them a call and see if you can special order it.
For the other two, there are EFK .40 barrels, but they are hard to get. I would definitely suggest a nickel plating, and then polishing that. More specifically, look at CCR's cera-plate of Robar's NP3 or NP3 Plus.
Really helpful links, found everything I want but the back strap. Inquiring with Beretta about that.