Just to reiterate, the cleats come with the pedals. So if you're buying used pedals, make sure you get the cleats as well or at least know how much they cost.
Not sure about matching cleats to shoes, since I run Shimano and have yet to see an MTB shoe that doesn't work with a standard 2-bolt cleat. You should be OK with most major brands.
(There are some cleat/shoe incompatibilities on the road side, but that's another story.)
"The song of a bird…We used to ask Ennesson to do bird calls. He could do them. How he could do them, and when he perished, along with him went all those birds…"-Return from the Stars, Stanislaw Lem
"We just walk around, and sometimes we go out and dance, and then we listen to the environment."-Ralf Hutter, Kraftwerk