[asid61]

You will probably need to broach your own. The only hex bore miter gear I know of is the Vex one. I would go with their 3/8" hex bore gears and have a reduction to the wheels to reduce the load on them.
SDP-SI sells 1/2" bore bevel gears, all you'd need to do is buy a hex broach and broach them. They also sell 12mm bore bevel gears (as does QTCgear), which you can bore out to 1/2" on a lathe relatively easily.
How much are you planning to spend on this? Swerve drives, even simple ones, are quite expensive.