Now... You don't have to find a Sonic Unleashed Blu-Ray disc for your PS3 anymore. Since yesterday, April 8, 2014 Sonic Unleashed has came into PlayStation Network as a downloadable game along with many other games like the PS2 classic Persona 4 as part of a PS Store Update. It's almost like the PS3 disc version but, it will cost you US$19.99, £15.99, AU$29.95, and you'll still be able to use the DLC that is available which is costs at US$1.99, and will require your PS3 at least 7.6 GB of memory with 1MB for Save Data. Although it's now on the US PlayStation Store, I don't know about others such as Europe, Japan, and others but feel free to comment if its there. Despite being available on the PS3, you still can't use it on the PS4 due to hardware compatibles (which the PS4 uses x86-64 instead of Cell architecture) although there's no word if the game will come back (via emulation on PlayStation Now or ported to the PS4) or in other cases be remastered like The Last Of Us which will have a remastered version of the PS3 game to the PS4 or even Sonic Adventure 2 on PSN or XBLA.
The link for the game for PS3 owners in the US and a few are right below (I don't have Canada, and a few but that's all I can find as of now), so feel free to get it if you don't want to just find the disc or you just can't find it at all.
As of always, you can still buy the disc version but it wont be easy than just buying it directly as a download.
For those Down under in Austrailia, and in the States and towards the UK, here's some links you can click to download right here online (and if you are on your PS3, go to the PS Store and search "Sonic Unleashed").