Milo, this forum has a few Walmart cheese fans. The few times I've bought it, it was excessively salty, but I'm sure it's produced regionally, so what you get may not be what I get. Just make sure you get the brick version and grate it yourself rather than the pre-grated stuff.
This forum also seems to like Polly-O. Last week I made two pies, one $5.89/lb Polly-O and one $2.99/lb Shop Rite brand (a major NE regional supermarket) and, although I have no photos, the results were identical. I have to admit that, in the past, Polly-O hasn't been that creamy, so the quality has improved, but it's still, imo, no better than supermarket brand cheese (which is phenomenal in my area). I'm not sure how much longer this will last for, but right now seems to be an amazing for mozzarella sales. Every week there's something on sale for as low as $1.77/lb. Due to last years complete dirth of sales during late spring and summer (ice cream season?), I don't think there's a lot of time left on these bargains.
I would keep your eye on the weekly circulars and get what's on sale. The only brick mozz that I constantly have problems with is Biazzo. Everything else works like a charm. When Sorrento drops below $2/lb I'll get that but most of the time it's house brand (Shop Rite, A&P, Foodtown, Pathmark, Acme, Stop & Shop, Wegmans). I believe that Walmart is almost always $2.99 where I am and the house brand is better and a buck cheaper on sale. A lot of pizzerias in the NY area shell out the extra money for Grande, but a lot use the cheap stuff, which is comparable to supermarket private label.
As far as stocking up on sale cheese goes... it's tough. Brick whole milk tends to get mushy over time, and, once this happens, it tends to curdle/go watery on the pizza. Part skim, on the other hand, keeps for much longer, but will burn faster on the pizza. It's hard to do, but when I reach for the packages on the shelf, I'll squeeze them and grab the firmest one. If they're already a little squishy, I won't buy it. Most of the time I can store brick mozz for 2 months without a problem. 3 months would be ideal, because, as I said, there are no cheese sales during the summer.