I ordered all of my lithiums from gearbest, fasttech, and ebay. Not within a week, took me many weeks to wait for good prices, apply points and wait to get them. Not the fastest way, but every single item was authentic so far.
Prices have gone up, that is obvious and shipping is no longer free either.

Walmart DOES sell 18650 and other form factors, but the ones I have seen were either not branded or unknown at a high price. So not a good choice.

another excellent source in the US is