Havent been there for years. We buy ours from Costco or Europea. Costco had a great offer on recently, we stocked up for the Boobs Cruise.
I do recall seeing a bus with a sign that said Walmart on it. It looked easy enough, but with no Spanish we opted not to try. We did take it to the market the other way though. I've been to Walmart once and took the very easy, less cost efficient way. Ask security to arrange a cab for you. They can help you negotiate a round trip price in advance, with the cab waiting for you while you shop. The bus is more cost effective.
I covered this in another thread, but the bus to Walmart stops right outside the resort. It is the "R2" bus (will be in BIG letters on the windshield and some even say walmart too). 10-12 min drive and stops about 50 yatds from the walmart entrance. The driver will let you know when to get off if you ask. On the way back, the R2 boards right atvthe walmart parking lot entrance and stops across the street from TTR. Cost for the ride is $1. Bring an extra set of underwear for the ride. Its quite the experience.
I have taken the bus there a few times, but always chicken out on bringing several bottles back with me on the bus and end up taking a taxi back.
I'm not sure, but if you've already passed immigration as you will have then it's not actually "duty free" so probably airport prices.