The first known tin bronzes seem to have appeared in the Caucasus region of Eurasia in about 5800 to 4600 BCE. That these very scarce early examples of tin bronze may have been accidentally made from rather rare ores that naturally contained both copper and tin simultaneously.
There is abundant evidence that by about 3000 BCE, tin bronzes were being made in the Aegean and Middle East (Turkey, Syria, Iraq, Iran) by deliberately alloying tin and copper, with the ores being obtained from separate sources.
I'm trying to imagine what the smelters/blacksmiths would have been doing.
They need surplus. If every bit of your product is in high demand, you don't have time/resources to tinker. But if you have some spare time, and your stock in trade is the (sometimes literally considered magical) transformation of rock into shiny metal, you're going to try other rocks to see what you get. Just a guess, but probably most of the tinkerers would be young men trying to prove for themselves what would work and what wouldn't. More experienced smiths would probably have developed rules of thumb about which rocks work and which don't--and the rules of thumb only worked for a subset of the ores they handled.
If you have a surplus, you can afford to "waste" a batch by mixing metals together. Maybe it will be just as good, probably it will be worse, every now and then it will be better.
But, and I'm guessing here, it doesn't seem likely that there would be a trade in rocks as such. So if you didn't have tin ore nearby, you would never try it for yourself.
One possibility is that a local smith who made stuff out of tin was given some trade copper whats-it to repair, and, not having enough copper left, filled up the lack with tin--and it worked. There was trade in copper ingots back in Greek times--that had to start somewhere.
Another is that the blacksmiths who learned to use the "joint" ore were recruited elsewhere, and finding the local ore only had part of what they needed, recruited traders to prospect around looking for the kind of rocks they thought they needed. ("Take this along with you, and if you find something that looks like it, bring back a vat's worth for me.") Once a good source was found... it was rare and valuable. "Evidence of tin trade in the Mediterranean can be seen in a number of Bronze Age shipwrecks containing tin ingots such as the Uluburun off the coast of Turkey dated 1300 BC which carried over 300 copper bars weighing 10 tons, and approximately 40 tin bars weighing 1 ton"--though that was almost a thousand years after that area had started exporting tin.
Once knowledge of tin's utility spread, it would spread faster within a civilization, but outside of a society it probably only spread by slow migration of blacksmiths who learned from blacksmiths. Especially in areas where the blacksmith was thought to be a magical person... Guessing.
It says here that copper smelting (as opposed to use of raw copper) appeared in South America circa 500-800 BC, but making tools didn't seem to be a priority and though there was a little arsenic bronze, there wasn't much bronze until about the time Cortez showed up. In North America raw copper was straightforward to come by, and smelting wasn't economical, so no sophisticated metallurgy developed. (Curiously, the Pacific NorthWest had iron--salvaged from wrecked Japanese ships.)
If most of the metal was ornamental, and the color was the most important aspect, it isn't obvious to me that the useful aspects of bronze would have been as obvious to the American smelters as it was to the Eurasian smelters.
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