We posted our big list of ammo sources online a while back to help you find ammo online, but I recently stumbled upon an awesome new site that helps you search for ammunition that’s in-stock and in real time called GunBot.net.
It searches various online retailers and shows up-to-date info on price and stock. You can search for various popular calibers as well as for magazines and reloading supplies. GunBot is still me beta mode so stay tuned for more features, calibers and accessories in the future.
Another similar site that I’ve had good luck with in the past is AmmoSeek.com. Their info is automatically sorted by cost.