This largely depends on how many you want to produce. I wouldn't recommend anything lower than say 100 females to start a colony with. Otherwise you're in for a very slow road.
You could either buy smaller sized dubia. They are cheaper and you get more per bug usually. Downside is you're going to have to wait a long time before they even turn into adults. Or you could get larger ones that are weeks/a month away from adults. You get less to start, but you start getting babies much quicker. Buying actual adults is hit and miss. Sometimes they are already older and die in a few months, not to mention most adult females are 75 cents to $1 a piece.
Patience is key here, to NOT feed anything out of the colony. You want to wait months for it to establish itself. If you need to feed dubia, buy feeders separately. Good luck!