You can use 'cause. It's ok. Some people use "cuz", " 'cos ", "cus" but it's slang.

yeah..usually in chatting we write it as "coz" or "cuz"

I agree with Peachey. If a person writes to me something like that, I'd think this person is really lazy to write words fully. I've got some messages like this: "hw r u", wich means "how are you". I think it's improperly.

I agree with Alice's answer, but I don't advise writing " 'cause " unless you are quoting someone. If you use it in your own writing, it looks simply lazy.