If your computer doesn't have a socket for an MMU or FPU already, you will probably need an accelerator board that includes such a socket, or you will need to replace your current chip with a chip that includes the missing feature (i.e. replace a MC68LC040 with an MC68040).
If you do have a socket (usually for an FPU, although some 68020-based computers will have sockets for both an MMU and FPU), you usually need to get an MMU or FPU that is rated at the same clock speed as your main processor (some boards may allow a different speed if they have multiple clocks).
If you have a 68020 and need an MMU, the 68551 MMU is the only choice. If you have a 68020 or 68030 and need an FPU, there are two choices: the 68881 and 68882; the 68882 has more FPU instructions on-board and is of a newer design, but it will be more expensive.
Most Amiga and Atari mail-order dealers will carry these chips; since these are standard Motorola chips, you don't need to buy them from a dealer for your system (just make sure you get one with the right speed rating). In the U.S., Paxtron is generally acknowledged to be the best source for new chips (particularly if you need Amiga custom chips); you may be able to find used ("pulled") chips elsewhere, including at online auctions and the like. At one time, Amiga International had some surplus 68882 FPU chips for sale as well.