I've used AmmoSeek.com a few times when I was searching for some hard-to-find calibers, mainly .45 ACP and 9mm. It's pretty handy as it compiles the prices from various vendors in one place, which saves a lot of time. I found some good deals, but I also noticed that some sites might not always have the stock they claim, so it's a good idea to double-check before you get your hopes up. I've seen a couple of sellers that felt a bit sketchy, so trust your gut and stick to the more established retailers when you can. Overall, it's a decent resource if you're cautious and do a bit of homework on the sellers.
