The best way to get .357 Maximum brass is to place a backorder with Starline.
The second "easiest" way to make .357 Maximum brass is to cut down .223 Remington cases, after expanding in small steps, up to .35 caliber. Then ream the cases to accept a .358" bullet. And, most importantly, spend a bunch of money modifying your firearm to use some type of moon clip (revolvers) or c-clips (single-shots).
A slightly cheaper way to get .357 Maximum brass is to have someone like Rocky Mountain Cartridge lathe-turn some cases from solid bar stock. You must, however, keep pressures quite low. The brass cannot be used in full pressure loads (not even remotely close to full pressure).
The worst way to make .357 Maximum brass is to cut down .357 Mag or .38 Special, ream the cases to accept an internal union (small piece of C260 tubing), and solder the union in place while also adding a case extension made from 3/8" C260 tubing. It has been done, and does work. But you must keep pressures fairly low, like with lathe-turned cases.
....Just give Starline a call, or place an order through their website.