Just ordered this one myself off Amazon.

TOOTO 5 PCS Male Straight Pneumatic 4mm M6 Ptfe Tube Push In Quick Connect Fitting PC4-M6 for Reprap 3D Printer Bowden Extruder https://www.amazon.com/dp/B019RQ5EYE/ref=cm_sw_r_cp_apa_8-oFAbYR6TNMT