Brad,
I first ran into the plastic push on fittings in a Sea Ray boat. But, they were metric! I think the brand was "Blue Whale" and imported from Europe. I could only get them at West Marine. My boat used 15mm!
All of the Lowe's stores in in the Seattle-Tacoma area carry them in plastic. I think the brand is "Watts". What Lowe's stocks may vary from state to state based on plumbing codes.
I can go to the local Lowe's and get the brand and SKUs if you strike out where you are.
Bill