Bouncing off of @RichardC 's excellent answer, there is the possibility that Wakanda has been clandestinely trading with the world for some time. The comics establish that Quinjets include technology from Wakanda. It is plausible that the equivalent MCU vehicles do too. It is possible that Quinjet tech was sold to the outside world through shell companies that concealed the true origins of the tech. I imagine it could be similar to how North Korea or Iran are able to buy and sell around sanctions and blockades, or how some organized crime and terror groups can "launder" funds coming in and out and seemingly function without obvious channels.
More mundanely, some of the overseas operatives could be working regular jobs in their host countries and sending portions of their incomes "back home" in the form of cash or cash equivalents. This is, in fact, a common practice in our own world, though mostly done electronically nowadays. We can see in Black Panther that Wakanda has cloaking devices, so how do we know that large quantities of US dollars, Euros, Yen, and other hard currencies are not being discretely exported (or smuggled out) weekly?