Pipe size does not have any logical relationship to actual dimensions.
3/8 schd 80 is commonly available at Home Depot, Lowes, & hardware stores. Typically as nipples rather than long sections of pipe.
Schd 80 refers to wall thickness. Be careful to not get the more common Schd 40. 3/8 pipe is .675" OD, schd 80 has a .423" ID