What is the difference between an AC-100 helmet and a PASGT?

I recently purchased an S10 respirator and like the SAS, I want a helmet to go with it. Now the SAS use an AC-100 helmet which is made for respirators and other SAS equipment but I cant seem to find one!!! So, I either need a website where i can purchase an AC-100 or the difference between an AC-100 and a PASGT. Can I use a PASGT instead of an AC-100?? are they basically the same?