Both the ASM and Uberti Walker and the Uberti 2nd Model Dragoon use nipples with a 1/4x28 thread. The Treso part number is 11-50-01. Track of the Wolf lists them with a catalog number of RST-A. The tip (or cone) height is 0.433" and they take a nipple wrench with a 0.215" slot width.
Here's the list of various nipple threads generally available with cross references between SAE and metric standards:
As far as cap sizes, to fit those Treso nipples I suggest Remington No. 11's. Some folks use CCI No. 11's, the only difference being they're taller, so they may require a bit more force to load to get them down on the cone far enough.