It's my understanding that the best pads incorporate both open and closed cell foam. I think you want the open cell in the initial impact area (i.e. the top) and the closed cell in the middle or on the bottom.
I just Googled "closed cell foam" and came up with all sorts of places to buy it online. I doubt you'll be able to find it in Prescott for a reasonable price; Phoenix or the inter-sphere are probably gonna be your best bets.
When you finish it, you should post some pics and helpful info that u learned along the way for others that might wanna make their own as well.
Maybe I'm mis-remembering, but I think most pad designs have a thin layer of closed cell foam on top and a thicker open cell layer below. That way the impact of a fall is spread out over a wider area of the pad. The closed cell helps distribute the force of a fall so it is caught by more of the squishy open cell layer making for a softer landing.