5/8" OD x 1/2" ID Black Tubing sold by Foot.
For use with 1/2" barbed fittings ( straight 13308, or elbow 12338 or 50TEF2088 ).

Has an I.D. of 1/2" and fits the 1/2" fittings listed as related products ( and a drain fitting comes with new valves purchased, in most cases ).

NOTE: This size tubing should be available locally where ever pipe and plumbing supplies are sold.