John Guest 1/4 in. O.D. x 1/2 in. MIP NPTF Polypropylene Push-to-Connect Adapter Fitting 803309 - The Home Depot
John Guest 3/8-in x 1/4-in Push-to-Connect Faucet Connector (10-Pack) in the Push to Connect Fittings department at Lowes.com
How to make a John Guest compressed air standard fitting connection - YouTube
John Guest Acetal Copolymer Push-To-Connect Tube Fitting, Reducing Branch Tee, 1/2" x 3/8" x 1/2" Tube OD (Pack of 10): Amazon.com: Industrial & Scientific