Sounds like you are trying to equate on O.D. with an I.D. measurement. Chances are a 3/8 steel pipe (if there is such a thing) will be larger than 1/2" O.D.
Try the Ace hardware, plumbing aisle. Some of those sink or toilet pipes are already chrome and may come close to the size you want.
If that doesn't work, I'll bet that Leanna can put a chrome powder coat on copper...
C