In the short run, the cheapest "local" place, IF you have one and IF they have any, is going to be your local WalMart.
Other than that, you are down to mail order or what ever local shops you have.
I say in the "short run", because in the "long run" the cheapest place is going to be the local shop you like doing business with. If you want to do a bulk ammo purchase with them, I'd wager they'll dicker price with you.
Once the local shop is gone (for lack of business from everybody looking for the "cheapest" place), the ONLY place will be WalMart or mail order ... and then I can assure you it won't be cheap.