Not rare at all, as long as you live in North America and don't want to buy it used. Amazon has it, Best Buy has it, Walmart has it, Gamestop has it . . . you name it. I've seen it hovering around $20 U.S. The used prices for the console versions are the same as the new price for the PC version.
Maybe you can't just walk into a store and expect to find it right away, but it can be ordered online from all of these places.
In the UK (where you live, according to your profile) the situation is different. The PC version was not released widely in stores. Game UK made a deal with Disney so that it would only be available for sale as a digital download over there. No retail copies.
But in North America it's fairly easy to get a hold of a retail version. You might also be able to get a retail version shipped from a place like Germany. The game comes with all the different languages (speech and text) on the disc, no matter which country it's sold in.