SPCA or humane society many times have a mobile unit where you can take her. Also talk to rescue groups in your area, they may have references. Some vets may do it at a lower cost or if you have a good one you know will sometimes help in finding her a home too once she has been spayed. I worked at one clinic where we would take in some strays from clients, have them signed over to us, and then we would feluk/fiv test, spay, vaccinate, deworm and adopt out.
Not sure if she has been tested or had any vaccines yet, but before spaying you may want to have her tested especially if she is going back outside after recovery. Feline leukemia and FIV are the two they test for and they are contagious among cats, so if she has either you would not want her back outside roaming. I know that is another expense though.
It never takes a cat long to get pregnant again. It is great that you have taken her under your wing to care for her. Hope you find a place quickly that can help you out. Here is a link for Arlington. If you google low cost spay neuter Arlington, Tx you may come up with more options. They will usually require vaccines to be current or given too. One other thing, sometimes you can get a coupon from a rescue, humane society etc for a regular vet's office that will make it low cost.
http://www.arlingtonhumanesociety.org/lowcost.html
http://www.lovethatcat.com/spayneuter.html#states#states (low cost to free if you qualify.)