Almost any ethnic grocery store will do since they cater to immigrant communities that are likely to be lower income. Here in SoCal some are cheaper than others but they're all way cheaper than Ralphs/Vons/Trader Joes/Costco/etc (I don't shop at WalMart so I'm not sure how they compare)

There are also native stores that are increasingly entering into low cost produce like Grocery Outlet and then there are the usual like Food4Less but they tend to eventually move upmarket.