I am so glad you posted this. I have the ACM M500 shotgun and you are right you have to search the deep reaches of the internet to find info and parts on it. My cocking arms broke on mine and I found where to buy them on airsoftgi.com just like you said but they are out of stock

. However, I have found another place to purchase them. You can get them at wgcshop.com however they are out of stock there as well and double the price. It is pretty frusterating. I guess all I can do is wait until they come back in stock somewhere.
Anyway, this is a really good guide and I recomend it to everyone who has this shotgun.