I do believe than those information can be acquire from net easily. I think, there is an ICS bible floating around somewhere.
Anyway, I will try my best to assist you.
Any buffer tube specify for AEG can be fit for ICS M4A1 with more or less modification. All you need to do is sand the stock adapter on both side to accept other stock. It might won't slide in easily but keep sanding it down until it slide in bit by bit to try out, the worst part is hammer can be used if you lose your patience.
I also attach two pictures to let you know where you have to sand. Hope it help.
I recommend you this buffer tube,
http://shop.ehobbyasia.com/d-boys-sp...or-m4-m16.html, they are great for the price plus it could fit a large battery in there.