I mostly get my coins from antique or charity shops here in the UK, or from online auctions. Happy to pay the Charity shops whatever they want, which can become a bit expensive for what I eventually get, but occasionally something interesting gets missed in a box or bag of mixed coins which I can then research. I've also found a local antique dealer who is only really interested in silver or gold coins, so happily hands over coins, medals or tokens of other metals at a reasonable price. I don't tend to spend much on my coins, apart from the occasional new issue from the Royal Mint, Pobjoy or the Westminster Collection, and enjoy finding interesting Exonumia as much as a new or different coin.
Other than that, I have an ever expanding list of friends who are happy to bring back circulation coins from the various countries they visit in return for BBQ invitations or a bottle of wine, as well as relatives and friends of my Dutch wife who keep an eye out for unusual Euro designs in their loose change. Not sure if any of that helps coin collectors in the USA, but it may spark an idea for an alternative source.
Good luck 😀