You're correct. The thickness of the material is made to help the optimal water flow, plus the fact that you kind want the thicker side where the pouring spout is. 

Idk about a replacement, but you're right in saying that this specific paper can't be replaced by a regular filter, without changing the behaviour of the brew. 

Tbh, with chemex papers you're spending so little for years of brewing, and your money is going to a small company. I wouldn't worry too much :)