I buy mine from mantispets.com. Cary always send good pupae. They come in great shape and fast. I wont buy maggots, they give my the creeps!
I buy 500 at a time because I feed them to my mantods so I will have to buy more often. I keep them in the fridge and take out what I need. There are other sellers of feeders online.
There are afew threads about raising your own bluebottles on Mantidforum.