I started my colony with a large mix pack from ABDragons. It took a few months for them to establish. If you want them to get going more quickly I would order some adults as well (they are more expensive). My colony is thriving several years later.
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.
I just bought 22 adult dubias (15 females & 7 males) on ebay for $25.00 shipped. I'm not sure if this was a good deal or not but it was the cheapest I could find. So ebay might be a good place to check out