Mario Kart Wii Platinum Soundtrack became available in 2011 as a platinum reward exclusively for the now discontinued Club Nintendo, It's one disc and has 43 tracks. I noticed that some of the track information was different on my CD than I've seen on others and so I found out that the titles vary depending on the N.A. Version, European Version and the direct Japanese translation. So, I choose the titles that sound the most natural out of the three—in case you're wondering why the track titles differ slightly.
You can still find copies floating around ebay ranging from about $60 - $80 in case you had some extra cash and wanted the physical copy but you'll find ours in the downloadable achive.