Most to all Phone network provider stores sell Pixels unlocked if you tell them you want handset only with no plan. If they get all pushy about phone plans then just say you are buying the phone for someone else, if they give a free SIM card just throw it away. These places take cash. This is likely a best bet to get the phone mint.
If you are concerned about buying straight from the mobile network then just get one refurbished/secondhand from a store like CeX or any other place, they always take cash.