Any "real" parts store should have them, and even Oh Reallys here has em. Brass cabinets in parts stores are typically color coded. "Compression fittings" Here, here's a catalog in pdf
EDITplews-edelmann.com/assets/1/7/AutomotiveFittings_90078.pdfEDIT
put
http://www. in front of the above to make the link. The damn thing wants to display it here as the entire file
Compression sleeves on on page 10. the "traditional" part no is 60 x "the size" which for 1/8 tube is 2 so "60 X 2"