Blatta Lateralis AKA Turkestan roaches get my vote as well if you need some at a certain size in a large quantity now. If you are looking for a good roach to use as the main feeder in the long term, get yourself a colony of Dubia (Blaptica Dubia) started and they will be ready in about 6-8 months for feeding from. Lobster roaches (Nauphoeta cinerea) would also be an ok choice, but they can climb glass so I tend to not like them very much.
http://bestroach.com He's a very helpful member here on the boards that I have heard nothing but great things about. I have yet to do business with him, but come next spring when I get into some new species, he is gonna get my business. I think he may have the Turkestans, not sure. You could send him a PM or email and see if he has some available for sale and in the size range you need. I do know he sales Dubia as well which is my ultimate long term recommendation on a roach species to use as a feeder.
http://theroachguy.com Very good guy, active and helpful on the forums as well. He only sales the Dubia roaches, but I have done business with him and know he is a good guy to order from.
http://aaronpauling.com/ This is another person I have heard good things about. I know he sales the lobsters and turkestans both, but the website says he is sold out of Turkestans.