If you don't feel you were treated fairly at Walmart, i would suggest contacting their headquarters to discuss it.
http://walmartstores.com/7663.aspx
The local Walmart has had fluctuating stocks of the more popular calibers of handgun ammo, but they don't appear to be hiding it. I did have to actually go behind the counter and point at which boxes i wanted with one particular non-gun employee. I have had some employees limit me to three boxes per purchase (regardless of rounds per box), but this does not seem to be a universally-known policy amongst the employees. I would appreciate more variety in their brands of ammo offered, but the store is more about "stacking deep and selling cheap" than offering offerings from each and every manufacturer.
As far as supporting my local gun shop, i've been sad to see and to hear about the gouging going on at some of the local shops. The one i purchased my most recently aquired pistols at didn't seem interested in selling any quantity of ammunition beyond a "test drive" box to go with a firearm sale.