Looking on the weekends is your problem. I go to Walmart every weekday before 11am and doing that can not only get 9mm but can afford to be selective about what I get. I pass on Federal 50rnd boxes as it's too much and wait to see Winchester White Box 100 round value packs, Tula (brass or steel), or Federal 100 round packs if I feel like getting my hands really dirty reloading mags (that stuff is dirtier than Tula steel cased).
The trick is definitely just to go every day. Contrary to many rumors most Walmarts don't get shipments on the same day every week or anything like that. At mine I have the most luck in Tuesdays but sometimes it appears on other days. Weekends are pointless unless they get a lot as it would be bought out.
Walmart is the right place to be looking to get prices that aren't jacked up. You just need to commit to going often and should buy whatever you can get (not more than you need though!) when you find it to hold you over when there's a dry week or two. Dry weeks do happen. Last month my Walmart had mountains of all the stuff I like. Like at times as much as 8000-10000 rounds of 9mm in stock. This month it's been scarce so far.