Lately, I exclusively use https://ammoseek.com. They don’t sell anything there; it’s an aggregator where people can summarize who’s got what available, price per round, purchase limits, and so on. Lots of filters you can apply to the search. So you can find, say, the cheapest remanufactured .45, or only brass-cased 62 grain 5.56, or specifically Hornady Critical Defense 9mm, or whatever. Then you choose the site from the results list, click the link to go to the listing on that site, and buy from them. If there’s a box of 50 9mm ball for $25 but with a limit of one per customer, I might go further down the list and choose one that will sell ten boxes at a time, otherwise I’m eating the cost of shipping on that one box instead of spreading the cost over several. Ground shipping doesn’t go up much between 2 pounds and 15.
I used to just run down to Dunham’s for a couple boxes of Blazer Brass for $12, but their shelves have been empty for two months and there’s no way I could keep track of who’s got how much of what for how much without a huge spreadsheet and a lot of spare time.