People often ask me where they can buy copies of my books - which is a tricky thing to answer because it depends on where you live and what you're looking for.
The first option is to try your local bookstore. Phone them, see what they have, and ask them to order if they don't. Support them. It's important.
If you want to order online in South Africa try loot.co.za.
If you're in the UK, try Waterstones.
For the US (and pretty much everywhere else), Amazon will have everything.