Almost all the blanks on the market are what they call "parade" blanks, they are lower powered then an actual grenade blank, and they are at least half the price.
If you want to make sure the gas block off is working, just throw the valve over and fire a round, if the bolt doesn't cycle it should be fine.
Any NATO grenade will fit, I forget the size.
The US ones that hold a regular grenade, you know like from WWII, do work but most of those adapters where sold to other countries years ago. They use to be dirt cheap but go for about $15 each now.
If you got the skill, you could make a tennis ball or golf ball launcher.