The cheapest way to get a 68882 FPU in the proper form factor: find an old Mac IIvx. No one wants them, and they're not considered collectible - you can get them for free if you look around.
The 68882 is soldered to the mobo - just slice it off with a utility knife, make sure none of the metal pins is sticking out from the ceramic core, and stick it in your LCII.
I've done this before and it works perfectly.
Best,
Matt