I have an LC III that has been overclocked to 33.3 MHz. I'm not sure what speed FPU I should buy - do I match the clock speed, or the bus speed? What kind of FPU should I put in this one?
Any 68882 in PLCC form factor should work if it's rated at least as fast as your main CPU (always use the effective CPU frequency, e.g. if you overclocked it). Since the 68882 is a static design, faster versions will work just fine.
Usually the last digit of the part number is the rated speed in MHz:
MC68882FN16 ^ ^ ^ ^ | | | rated 16 MHz | | PLCC | 68882 Manufactured by Motorola
There are some third party manufacturers, as well (e.g. Atmel), their naming works similarly: TS68882MRB/C25 is a mil-spec 25 MHz chip in a PGA package (i.e. it won't fit your LC III), but be aware that the designation for the package and spec is specific for the manufacturer. If in doubt, google the datasheet and make sure the values match.
You could even use a 68881 (they're pin-compatible), but it will be about 60% slower.