I think you can buy transition sunglasses at any optical store including walmart. However, I don't know the price of the sunglasses in walmart. You can either go to their chain stores at your local places to get more information or just enter their website to get the detailed information you want to know.
Yes, they offer transitions sunglasses. The costs differs according to the frames you choose for your glasses, usually the price of frames at Walmart ranges from $35-200. The lenses for transition may start at $120. You can search online, you may find it is much cheaper than that of Walmart.