Certified GKH Gardening Expert
Fall and spring is the best time to seed your lawn, though to be honest, with the drought that you guys are having, I would wait to reseed. The reason is that it will be very difficult for the grass to establish in such conditions. You would essentially be throwing your money on the ground as grass does best in establishing with lots of water available. That is why you wait until spring or fall. Ideal temps with higher rainfall, normally.
I know it is hard to look at a sparse lawn, but wait until the weather improves before seeding. As for purchasing, try local nurseries or even feed stores. If that does not work, there are several online suppliers that can help you.