Unfortunately their is not a selective product that will remove crabgrass from St. Augustine. St. Augustine is a very fragile grass, and so most selective products that are labeled to remove crabgrass and not your real grass, will hurt the St. Augustine. It is recommended to kill the crabgrass with a round up type product, like Ranger Pro, which will also kill just the St. Augustine around the crabgrass that it is sprayed on, and then a couple weeks later re-seed those areas with new St. Augustine.
Ranger Pro: http://www.domyownpestcontrol.com/ranger-pro-herbicide-25-gallons-p-1124.html
For long term control though, you should apply a pre-emergent in the late fall and early spring which will prevent new crabgrass from coming up next year. If you apply pre-emergent every year, you should not have issues with crabgrass long term. Barricade is one of our most popular pre-emergents for crabgrass.
Barricade: http://www.domyownpestcontrol.com/barricade-65-wdg-herbicide-p-1491.html
Ranger Pro is a non selective post emergent herbicide and it will kill just about any type of vegetation that it is sprayed on so we would not advise for you to use on your lawn.
Our most popular general weed control product that is selective, meaning it will not harm your turf grass but it will kill your broadleaf weeds, is End Run. End Run is labeled for just about any weed you could have and will not harm your grass. Best of all, it comes with a surfactant already mixed in the product so you do not have to purchase and mix a surfactant separately like you do with most of the other herbicides that we sell.