Your question is imprecise in that money conversion experiences will vary greatly depending on where in the world you are. Some will be reliable and trustworthy, and others could be very risky (to you).<p>As GFK mentioned, an ATM is the way to go if it's an option. Then at least it's just your bank potentially screwing you. And if you are converting western money to cheap third world money, you probably don't care about losing 10% instead of 3%.<p>My recommendation: if there's no obvious ATM option, go to the best looking money exchange office you can find. Then transfer enough to get you through a day or two. And during that day or two, do whatever (internet) research you can to find out the best local options.
Revolut is pretty good at this, you can convert money in-app at pretty good rates (similar to what you would find on Google). You also get a Mastercard and everything.