I had the same situation once and all I had to do was look around in the basement and I found a section that WAS copper. Typically, there are some small sections of copper with these plastic piped houses. Maybe look near the washing machine, hot water heater or near the main inlet area. You DON'T want hot water and you don't want to be too close to an adjoining fitting. If all else fails I bet that you can purchase a fitting that could be put in-line in your plumbing that had a "y" already pre-formed for your 1/4" outlet.