Most TA's don't want to book SW. SW does not pay any commission and at least last time I heard they were not on any TA booking sites. For example we use the Amex Travel Service at work and we have to do our own SW booking because they don't even show on the screen. So I would expect a TA to be reluctant to book this and probably to charge you. (Of course don't most of them charge for plane booking now?)
The only time I've heard of a TA booking SW was the company we used to work for, their corporate TA would book them for flights within Texas because they were so cheap.