Smashwords
1. You can purchase from Smashwords from anywhere in the world as long as you have internet and a credit card.
2. When you buy a book, you can download multiple formats for whatever you read on.
And those are the reasons why I will always advise you to purchase from Smashwords over any other store. (There are actually many more reasons, but those are the big ones.)
Apple
Apple is the second biggest international store. There are more iDevices (iPad, iPhone, etc) than any other type of reader out there.
Argentina, Australia, Austria, Belgium, Bolivia, Brazil, Bulgaria, Canada, Chile, Colombia, Costa Rica, Cyprus, Czech Republic, Denmark, Dominican Republic, Ecuador, El Salvador, Estonia, Finland, France, Germany, Greece Guatemala, Honduras, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Mexico, Netherlands, New Zealand, Nicaragua, Norway, Panama, Paraguay, Peru, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, United States, United Kingdom, and Venezuela.
Kobo - A Canadian company that was recently bought out by Rakatan, a Japanese company.
Honestly, if you're living in any of these countries, I'd recommend the Kobo reader over Amazon's Kindle. Kobo gives every evidence of being a better company internationally than Amazon. This list states that it's as of May 2012, but they distribute to Japan now too.
As of May 2012, our list of established territories includes:
Canada, United States, United Kingdom, Australia, New Zealand, Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Singapore, Hong Kong
Amazon
Amazon is working hard to become the world's biggest online retailer. They are very likely to succeed. However, they tend to be combative and are actively trying to avoid taxes wherever possible. This makes their entry into the international market difficult as they are pissing off governments from what I can see. That said, they will succeed. These are the countries where they have stores that are reported to me.
It is my understanding that they distribute to many more countries, but they add a $2.00 surcharge to each book. Also, I don't receive the normal 70% royalty in these countries. Amazon only pays 35%. They are the only company that cuts royalties from other countries. Amazon claims that they make books available in Canada and more than 175 countries worldwide.
Andorra, Austria, Belgium, Brazil, Canada, France, Great Britain, Guernsey, Germany, Italy, India, Isle of Man, Japan, Jersey, Lichtenstein, Luxembourg, Monaco, San Marino, Switzerland, Spain, United States, Vatican City
Like I said, they claim to distribute to 175 countries worldwide, but if I understand correctly (and I may be mistaken) people have to log onto one of the other Amazon stores and they pay a surcharge for the books.
Barnes and Noble
It used to be that Barnes and Noble was only available in the U.S., but they have now expanded to the U.K. and are partnering with brick and mortar bookstores to sell their ereader. After an infusion of cash from Microsoft, they are working on establishing a presence in new countries as well.
For three others, I contacted Smashwords and this is the information I received.
Sony
Has the US and Canada, though I hear they may be expanding into
other markets.
Diesel
Doesn't consistently provide location figures, but has included
Australia, New Zealand, India, the UK, Canada, Mexico and several
African nations.
Blio
Predominantly has US sales, but also UK, Canada, Mexico and New
Zealand.