Almost every Costco I've been to, the food vending is a window on the outside of the building (so before the checkpoint).

And, at the ones where the food vending is on the inside, it's on the "paid" side of the cash registers. You can get to this area just by claiming you're "going to member services", since that's also on that side, and it's where you go when you forget your card (and so can't show membership at the checkpoint).

Source: Have forgotten my card a lot.