https://www.milwaukeetool.com/power-tools/cordless
everything here that is listed as either M18 (the boxy slide in battery) or M12 (the cylindrical battery).
M28 is something different.
in general, tool numbers are usually NNNN-BB(LL)
where NNNN is the model number, BB indicates bare tool (20), or kit (tool, battery, and charger (various numbers greater than 20, indicating either number of batteries, or capacity of batteries)). LL is also optionally used to indicate things like capacity of batteries.
sometimes you will see a BB of 80. this seems to be used only by CPOTOOLS.com, to indicate refurbished tools.
if you are looking for additional batteries, look on ebay, prices there are the best, but make sure the battery says "milwaukee" on it. there are a lot of clones out there. when you search for batteries ("milwaukee M18 battery" will get you started until you get a handle of the model numbers, which are a completely different format from the tools), there is a check box on the left hand side labeled "milwaukee". select that, and it will get rid of a bunch of the off branded batteries, but not all, so be careful.
batteries of both styles are available in a large amount of capacities, so make sure you are comparing similar batteries. as an example, a 5AH (amp hour) M18 battery is likely worth 5$ more than a 3AH one, since it has almost twice the capacity.
interestingly, there does not seem to be a lot of logic in the pricing of batteries on ebay, prices vary wildly, while tools dont seem to fluctuate that much.
good luck. it is a very frustrating place to play, since model numbers dont seem to have structure that allows you to quickly grasp what you are looking at, but they are pretty nice tools, and seem worth the effort
