.218 Bee is one of those cartridges that simply needs to be handloaded for. It's almost like 80+ years ago again when the .218 Bee was a cute little wildcat cartridge with no factory support. You're in the same boat as we who shoot .30-40 Krags, .219 Zippers, and .32 Remingtons, etc.- until recently factory ammo was by and large available, but now demand the skills of a handloader to keep running.
If you're not a handloader, now's the time to start. .25-20 brass can be found if you are diligent- the basis for the .218 Bee. I recently bought a .25-20 single shot and had dies, bullets, and brass in hand inside of a week after making a couple appeals in internet classifieds.
Neat gun. I envy you your Browning lever .218.