Hit these links:
http://www.bikeworld.com/local/maps/mtbtrails.pdf
O.P. SCHNABLE PARK
This is a fun place to ride with a
variety of trails. Within a short ride
from the start are majestic 40 ft cliffs
overlooking a dry stream. You can
ride around at stream level or take
the trail to the top of the cliff to a
scenic overlook. If you go far enough,
some of these trails connect with
others that criss-cross San Antonio's
drainage areas.
From 410 go north on Hwy 16
(Bandera Rd) 4.5 miles and turn right
McALLISTER PARK
Located on the north side near the
airport. It's closeness and variety of
trails makes it a favorite for all levels
of riders. For the beginner there are
several asphalt trails that give you a
taste of the wild, and for the expert
there is plenty of tight, fast single
track with some technical climbing
sprinkled in.
From 281 north, take Nakoma exit.
Turn right on Nakoma and follow this
road about a half mile past the
stadium. The park entrance is on the
right.
Group (I've never ridden with them, but found the link)
http://www.storm-web.org/