I agree with BurBunny that unless you have a relationship with a global network like HSBC Premier, using the ATM is far and away the cheapest way to go. Most ATMs will use the same exchange rate (interbank + 2%), but some cards will waive the ATM fee for using a foreign ATM which can be pricey. You mentioned that you exchange $ with B of A...if you have a B of A ATM card google "Global ATM alliance" - IIRC they partner with Barclays in RSA and if you use one of their machines it should be with no extra fee. Many credit unions and places like Schwab, Cap 1, Fidelity also have this policy. The only dodgy thing is that the foreign ATM must list the ATM fee as a separate line item to get it waived instead of bundling it into the dispensed amount. Check w/your ATM card provider first & also let them know you'll be traveling overseas.

I do several transit flights a year (e.g., US > NZ change plane> AUS) and to cover expenses like a bottle of water or candy at the airport I use a credit card -- might have a $5 minimum purchase but then I'll just add another candy bar, cheaper than getting kiwi cash. As an alternative I always carry a bunch of $1 bills - I haven't found a gift shop at a gateway airport that doesn't have a cheat sheet for exchanging USD, GBP, Euros, etc. and if it's not my destination country they can just keep the change, hate having a few rogue coins which make me look stupid when I accidentally try to use them.