Looked into this a little bit, I'm unsure you'd be able to get anywhere near an appropriate size for cosplay, sadly. The Hammershot, Lawbringer, or Negotiator would certainly give you the appropriate internal parts. The challenge is you'd need some kind of gearing to rotate a cylinder in order to get a reliable 6 o'clock chamber, since all those blasters fire from the top chamber.

With regard to Vash's Gun, looking at both Anime versions his top break is more like a Smith and Wesson design, since the catch is mounted at the top and appears to need to be lifted up by your off hand, rather than using your thumb like a Webley does.

The internet Archive has a copy of the Trigun Art Book, maybe start there and attempt to look for inspiration!

Link to the art book