Question about prepaid & pay as you go phone service?

I've been researching to find a phone carrier where I can pay only for the minutes, text, or data that I use-no stipulations involved. But it seems with all these no contract carriers and plans you have to at least buy more minutes after a certain period of time, whether it be every month, 3 months or 6 regardless of if you still have available minutes or not. I want to know if anyone knows of a carrier available in Houston, TX. where I can use the minutes that I paid for whenever I want. Whether if I want to use them all up in 5 days or 5 months. So please if anyone knows help me out.