Nintendo Eshop

all 'Nintendo of Europe' countries (which includes South Africa) are easy to buy from as UK cards will work no problem, use a fee-free card as most cards have a foreign currency transaction fee.

South American countries are very difficult to buy from, so can just ignore those.

If you want to buy from US then you might want to use a separate account to add US eShop top up to, as people have found themselves unable to revert their account country whilst still having any amount of foreign currency credit remaining on their account.
I imagine the same can likely be said for Japan, and maybe some other countries.

If you are just redeeming a code for a game though, then that won't cause any problems with reverting your account country

As already mentioned eshop-prices is the website to always check, be aware that it doesn't always update all the countries' prices immediately so look down the list to see if some aren't showing the current discount yet

Another good website to use is dekudeals, it doesn't shop all the different countries' prices at once like eshop-prices, but it is more useful for seeing exactly when and how a often a game has been discounted
You can change country at the top of the webpage via a drop-down menu