There are many internet forums (like this one) that have forum rooms set aside for people to post offers to buy/sell gold. There is also ebay.com and craigslist.org, but there is a lot of fraud out there, so be careful.
I just returned from Ecuador.
In the cities and towns I traveled in, not a coin dealer in sight.
I stopped in the largest market in South America, no coin dealer minimal coins.
Found coins only in jewelry shops.
Walked in, asked, and watched them dig coins out of drawers for me to select from.
Similar approach for 24k jewelry to wear home.
Cash talks, that is for sure.
Got a small digital scale, bring it.
I spent some time in Costa Rica a few years ago and I don't recall ever seeing a coin shop down there either (though, to be honest, I wasn't looking for one at the time). They do have a fascinating gold museum though with a lot of pre-Columbian gold jewelry/art. It's amazing how they were able to work gold back then with very rudimentary tools.