I do reload for .223, .45, and 308 but to me 9mm is right on the cusp of worth the time. Again just my opinion. I have looked into casting but have not taken the plunge.
As for Walmart and inventory, they probably get fulfilled from the vendor based on inventory rather than ordering themselves. Not sure if that is the case for ammo but I know that is the case for other products. If the system shows they have some in stock when they don't, two things occur. The buyers see that it is not moving and don't have time or bother to order more and the vendor sees they have many days on hand and does not push more to the DC for the store.
Again not sure if that is the case here but it never hurts to ask what the system shows on hand ( if you can find someone ). If the system is not correct, finding someone to correct it is even more difficult.