If you have more time than money, you can form 38-55 brass from 30-30 brass. The cases will be a little short, but that really doesn't matter, since you probably won't be roll crimping your BP loads. Just seat the bullet out to the correct over all length for your load, and your powder capacity won't suffer, either. A little scrounging at your local shooting range might even provide you with a free source of 30-30 brass.
Ken Waters makes 38-55 brass out of 30-30 and 32 Special brass by putting 15 grains of Bullseye in the case, topped with a small twist of paper towel or toilet tissue BUT NO BULLET. You fire this load into the air, to make sure the powder and wad stay at the primer end of the case. Every time you pull the trigger, you get a fire formed 38-55 case that's just a little bit short. He says this is quicker and easier than sizing in stages with dies. Your brass will certainly never grow too long for your chamber!
