Probably cheapest place if you just want adults is a reptile expo in your area.
I buy my dubias (hopefully done buying them) from dubideli.com. They have good prices and ship pretty quickly. You can get a set of 10 females and 3 males for $10. This would be plenty to start a colony for 1 dragon. You will probably have to pay $7 shipping, so you might as well buy some appropriate sized nymphs to start feeding your dragon right away.