Check out the frozen food sections at HEB and Randall's - I've sometimes seen bison and even venison there. (I'm pretty sure bison shows up from time to time at the regular meat counters, too . . . I'm
sure one of these stores was sampling bison burgers once.)
There's Whole Foods Market, too, although since it's popularly known as "HolierThanThou Foods Market," they may not carry game meat by choice.
If you drive out to Lakeway once in a while (early AM is best), you can almost certainly see some fresh-killed venison by the roadside . . . it's not too far west of Hudson's, and there are usually a couple of roadkill deer per week along 620 and/or Lakeway Boulevard; if you get to them before animal control does . . .