the only place ive ever seen them for sale is my old school's science lab. they kept a case full for biology study and when there were too many they sold them for 50p each.
You get eggs sold as fish food on ebay which I think are easy to hatch. Also your local reptile store should have the contacts to get you what you need. I f you give us a rough location we could maybe be more helpful
i kept stickinsects when I was in school! I started off with three ended up with well over a hundred! The eggs took about a year to hatch and they laid tons of eggs! Mine were just the ones that ate Privet hedge! No idea about the larger ones though!
Make pretty cool pets though as they don't mind being held and they're not too quick either!