The soundtrack for Mario Kart 8 was released in June exclusively to Club Nintendo members of Japan, Europe and Australia. If you're anywhere else in the world(like me), you're best bet for scoring a physical copy is ebay. Yeah, It's been a year since MK8 was released but considering how good the soundtrack is, it's better late than never.
Like many of you, my friends and I spent most of the summer last year playing Mario Kart 8 and I remember at the time I couldn't find any info for an official soundtrack release, so I recorded most of the songs from the game itself but now since I have the official soundtrack I uploaded it in place of the ones I recorded. They're much better quality and there were a few tracks I missed recording anyway. There are some sweet remixes of old classics and plenty of new songs included, it's a really good listen.
The album's uploaded and you'll find it on the download page. Enjoy!