Does anybody know where I could order the one-piece solid gel pad for a ACH?

I recommend giving them a call at Ranger Joe's [login to see] . They've always been able to help me when I was looking for some specific kit. Give em a call, don't forget to ask them to send out the new catalogue. They speak our lingo and can help you find exactly what your after.

http://www.rangerjoes.com/Default.aspx

I can't help you with that particular product, but if you want to make your ACH more comfortable, look up Oregon Aero. Make sure that you size your pads correctly as well. A lot of helmet discomfort can be traced to the fact that supply sergeants always give out size 6 pads, even though there are size 4 and 8. For example, I have a 7 1/4 hat size. I require a large helmet with size 8 pads, or a medium with size 4, if I remember correctly. I switched to Oregon Aero size 4 pads in my medium helmet, and my headaches immediately went away.


There's a possibility that this is the product you are thinking of.
http://www.oregonaero.com/helmet-upgrades/zeta-ii-helmet-liner

I believe you are referring to this product here:
https://www.teamwendy.com/products/aftermarket-systems/liner-systems/epic-combat-helmet-liner-system

Beware: a lot of senior leaders frown on after-market OCIE mods. Not saying it is wrong, just saying.