Does anyone know where I can get a PDS plug-in 68881 maths coprocessor card fo a LC? I have 68882 cards for a LCII, but these don't work on the LC.

You might see if your local mac shop still has one
in their junk bin, if they havent already thrown
away all their 68K stuff (most have).
Otherwise, try Ebay..in the past i've had some luck
finding the most obscure old mac parts there, as
there's always someone trying to sell really old
68K mac stuff, and often cheap as there's
rarely any bid competition anymore..
It'll probably still cost you more, after shipping etc,
than the maybe 10 bucks or so for a used LC
475, or something similar, which, although it
doesn't have a co-processer either (no LC's do), is
much much faster and can take 128 megs of
ram...
There's tons of others models of 68K , too for
about that much these days , some with math
chips in them too...most are really cheap
nowadays...
Of the old 68K macs, only the historically
significant ones such as the very first Macs, the
Color Classics, and SE30's go for larger sums...