.454/.45lc
I'd suggest, in order:
1. .454 Casull/.45 LC - practice with the .45LC, starting with Cowboy Action Shooting loads, moving up. The LC loads will be less expensive and are likely available around here without problems. .45LC would also be more than adequate for home defense. Buy the Casull rounds and try them out eventually.
2. .44Mag/.44 Special - practice with the .44 Special, assuming you can find any. Should also be some reduced loadings of .44 Magnum available somewhere, though you might have to special or mail-order them.
3. .480 - Practice is important, and you likely won't want to endure the physical or fiscal costs of practicing with this.