This is one of those times when eBay is your friend.
NuBUS Ethernet cards,
especially out of productions ones tend to go pretty cheaply. Should also
be possible to find 10Bas2 with no great difficulty.
Just look for something like a Kinetics/Shiva/Dayna EtherPORT/DaynaPORT or
one of the old Apple ones (a rebadged 3Com 3C5x3). Apple's is supported
by standard system software. 3Com driver is still on the website. The
Kinetics and Shiva are supported by teh Dayna driver; Dayna's web site
still being up the last time I looked (if not, I have a copy of the
installer).
Old cheap coax (10b2) cards aren't hard to find, but I think in most cases
another $5 or so for a newer buffered card is worthwhile (like the Asante
or Farallon). Most people I offer the combo cards (10bt and 10b2) prefer
that option over either of the single function cards too. When you can get
a brand new in the box Asante or Farallon for $20, the savings on a cheaper
card seem to have less value to me. (Asante and Farallon run fine with
Apple software, and both have tech support that is GREAT).
I'm not sure of this, but I think the Dayna cards driver has some problem
with newer OS releases (likely not an issue except in a nubus PPC).