I started off my colony buying from Petco since I did not have any other option. I first bough 2 females (Small they were about 100 grams) and paid $4.99 each for them.
I raised them up and bought a male from Petsmart.
They started to mate when the females reached 250 grams.
Since than I have bought other females but mainly held back some that I produced.
I no longer buy females (I just old them back) and for the males I will either buy them when the one I have retire or trade with people that are local to me.