My Mirouku built 1855 Highwall slugs out at 0.377 groove diameter. I use bullets ranging from 0.379 - 0.380 in diameter, which are readily available in cast, Hi-Tek and Polymer Coated forms in various weights. Badman Bullets, Cheycast Bullets, Bearcreek Supply are just three examples. I've used all three in addition to home-cast bullets found in an estate sale. In the latter case, my buddy sizes and lubes them at 0.379 for me.
Go to Starline Brass website and read the tech note they have on the .38-55 wcf cartridge for more information, but essentially, the best answer is to slug your own barrel to confirm what your gun's groove diameter is. In cases of the .38-55 wcf guns, vintage up to modern production, they are not all equal and have a wide range of differing groove diameters. Slug your barrel first, then go bullet hunting.