Here are just a few of the stores I frequent or have found great things at:

Otra Vez Consignment- An independent consignment store with very friendly staff. They have high end designer items here inculding some petite items.

Buffalo Exchange- Actually right across the street from Otra Vez, I have found petite and XS clothing here too.

We have several malls and shopping centers with the usual Ann Taylor, Banana Republic, etc. There are plenty of stores at all of these places but I am just putting petite specific places.

La Cantera Mall- A very pretty outdoor mall with
Ann Taylor Loft
Ann Taylor
Nordstrom
Neiman Marcus

The Rim- Just down the street from La Cantera
Nordstrom Rack
Saks Off 5th

The Quarry- Another nice outdoor shopping area close to the SA airport (if you have a layover in SA, go here! lol)
Ann Taylor Loft
Ann Taylor
Banana Republic Petites

Northstar Mall- A few miles away from the Quarry and also close to the SA airport
Ann Taylor

The Village at Stone Oak- Yet another outdoor shopping center off 281/Stone Oak
Ann Taylor Loft
DSW (near the Target) They have the biggest size 5 and under clearance section than the other two in the city

Tanger Outlets in San Marcos- About an 45 min drive from San Antonio
Ann Taylor Loft
Banana Republic Petites
Last Call Neiman Marcus
Barneys Co/op
Seven for All Mankind
Saks off 5th