Kindle for Mac application by Amazon can also be used for purchasing Kindle Store books outside the USA.

How to purchase Kindle Store books outside the USA using Kindle for Mac

1. Download the Kindle for Mac application from the Amazon and install it on your Macintosh. It's free.
2. Create a new account with Amazon.com. Use a US address as your delivery address. You can use a virtual USA address from companies like Shipito.
3. Sign in to the Kindle for Mac with your new account.
4. If you have a US debit or credit card, you can pay for books from US Kindle Store with it.
   
   If you don't have a US debit or credit card, just buy an Amazon.com Gift Card (e-mail gift card or printable gift card) from your non-US Amazon account. You can also buy Amazon Gift cards on eBay.
   
5. Select a Kindle book. On the payment page, enter your Gift Card. Your payment will be processed and your ebook will be downloaded the next time you sync your Kindle for Mac. Your purchased book will appear in the "Archive" section on the Kindle for Mac.
   
   Note that Amazon can reject your transaction if you use a non-US IP address. You can get one by using a US proxy server, a US VPN or Tor (note that using Tor is risky).

Alternatively you can try to use the "Assisted purchases" service from Shipito or other companies.