The rtl8191su does depend on non-free firmware so it definitely will not work
with Trisquel. The chipsets that will work are primarily the rtl8187b and
rtl8187l. There is also an ar9170 that I don't recommend. We sell these cards
and have thoroughly researched the best chipsets. We use them with Trisquel
in-house.
[http://libre.thinkpenguin.com/ ThinkPenguin.com]
It is unlikely you can get them locally and few places online either. We
import them from Taiwan and I know both the rtl8187b and ar9170 chipsets are
in very very few adapters any more. What adapters did have them have or are
being discontinued.
My advise is to check out the
[http://en.wikipedia.org/wiki/Comparison_of_open-source_wireless_drivers
wikipedia article ] on wireless chipsets.
