The Denton Jazz Festival is a HUGE event every year in Denton. It is free to the public and the booths fees are reasonable (In my opinion).. I have access to a tent, table, chairs, all I would really need are signs and I have brochures coming (though I would need more). And I bet I could wrangle someone to do blood pressure checks if that is something that is acceptable.
Also, the Denton Dogs Days of Summer is a good one
Here are some other links for events around Texas
There are also Balloon Festivels, Highland Village and Plano have them. Highland Village also has a Red, White and Blues event in June.