All you have to do is send them an email requesting they carry a certain product.
I do it all the time. A while back I requested a certain brisket marinade that was hard to find. 2 weeks later they stocked it for me. I requested a certain hot sauce as well but that hasn't come through yet.
I have already sent a email requesting they stock vitaminfizz. Same as last time they answerd my email and said they will forward my request to the purchasing specialist.
WM will sell anything that they think they can make pennies on. If you don't believe that, take a walk down the aisle directly in front of the cash registers. Know what's in those big boxes and cages? Products that they thought they could sell but didn't. It a win - win for WalMart. Not only do they make $$ off of the product but they might also charge slotting fees which is what they charge the company for using their shelf space.