http://www.netgate.com has some 'Soekris Kit' solutions that screw right in.
I placed one order with them and it shipped fast and was exactly what I
expected. It wasn't the cheapest source of Senoa cards (but still
reasonable). and it was convenient to get everything together for that
particular order.

I'm sure they're not the only source, however, just the one I've used and
was happy with. YMMV, etc. :-)

cheers, michael