Where can I buy Apple Watch Series 5 - Model A2156 in the USA?

There isn't any authorized source for it in the USA. If you choose to buy a secondhand one, ensure that it's not activation-locked before giving the seller your money.

(178575)

Hi ajeep8u

also if you buy 2nd hand I would recommend eBay because it is against ebay's terms of service to sell any apple device that is iCloud locked so you will always get your money back if you complain directly to them