This soundtrack can be purchased at the following sites:
I really loved this game and it's soundtrack, and it's not surprising that it has received so many fans and covers of the music.
The OST is in one word, great. Not only is it a whopping 101 tracks (the iTunes version), but it's also very varied. Just like the game, the OST has a fun quirkiness to it; take the track called 'Can You Really Call This A Hotel, I Didn't Receive A Mint On My Pillow Or Anything' for example, which is played as you dodge the attacks of an insane showhost-robot.
Tracks range from short interludes to 6 minute compositions; the themes are wildly varied. Every character has it's own theme, which can be happy, melancholic, jazzy, bittersweet and more.
One of the most covered songs of the OST, Megalovania, is only played during a 'genocide' run of the game. I can recommend watching a LP of it, as it's the most time-consuming achievement of the game.
There are too many favorites tracks to list, so i'll only add a handful to this website; the rest you will have to check out for yourself.