One Option: Airvoice uses At+t phones and has affordable plans.
Coverage should be the same as your current At+t plan.
Plans:
$10 a'la carte plan (2 cent texts, 4 cents per minute talk, 7 cents mb data), Lasts one month.
$30 unlimited talk/text with 500 mb data
Website is basic. It does have auto monthly renews if you like.