Super Serial Card II $20
Complete with a copy of the manual containing
instructions for dipswitch settings
I don't know of an online source, but I can point to an offline one:
The original version of the manual for the SSC (dating back to the Apple
][+ era) has detailed information on programming the card, including a
summary of the 6551 ACIA and a firmware listing. The later edition of
the manual (from the IIe era) removed all the low-level information.
Getting a copy of hte data sheet for the Rockwell 6551 would also be a
good place to start, as this explains how the ACIA chip operates. All
you need to know in addition to this is how the chip is interfaced to
the Apple II: I/O locations, DIP switches, interrupt behaviour, and the
external mapping of signals to the ACIA pins.
2. GNU Project
7. SSC and a GS
12. SSC DIP Switches