The 307 is a rimmed version of the 308. Years ago I almost bought one, a custom 94, but one of the salesmen at the store bought it while I was researching it.
IIRC, it uses the same dies to reload as the 308 with a different shell holder. Only use 307 loads because it is loaded to a slightly lower pressure than the 308. Bullet selection is a little tricky because any bullets suitable for the tubular mag will have been designed for the 30-30 and about 400 fps less MV. So be sure to use the toughest bullets you can find.
Brass and ammo is a seasonal item, which means that Winchester cranks out a run every couple of years whenever they feel like it. The good news is that both are currently available, so stock up NOW.
Winchester Brass 307 Winchester - MidwayUSA
Winchester Super-X Ammunition 307 Winchester 180 Grain Power-Point Box of 20 - MidwayUSA