Brick and Mortar shops all over the western half of Texas. Online, the usual suspects: Grafs, Midway, Widner's, Powder Valley, Precision Reloading, Coyote Armory (though they are kind of local to me).
Good thing about online, you can see if the dealer has any. Drawback is you'll need to buy in larger quantities to ease the pain of the Haz-Mat (hazardous Materials) fee they all will charge - usually $26-30 per shipment.
But, you can combine several pounds of powder and a few thousand primers in most shipments.