The availability of Tsum Tsums can vary from country to country; however, Disney is steadily adding more and more to the range and increasing availability.
Currently you can find Tsum Tsums at the US Disney Store (online as well as in store), European Disney Stores, the US Disney Parks (at the World of Disney stores), Japanese Disney Stores, Hong Kong Disneyland, Disneyland Paris, Clinton's in the UK and are coming soon to the Disney Store UK. A limited number of characters have also recently appeared at Target in the US.
Tokyo, where Tsum Tsums started, tends to get the greatest variety, and nearly always get seasonal variations.
Disneyland Paris seems to have one core range of them, including some that are not available in the US yet like the Three Little Pigs.
Hong Kong Disneyland seems to get what Japan has, but not as many, for example they got the Halloween ones from Japanese Disney Store while the US did not.
If you are looking for Japanese exclusive Tsum Tsums we have had success with Disney Dream Store from Japan. That is how we get most of our Japanese Tsum Tsums!