Hello, and welcome to the forums!
Sorry to hear that Mafia 2 is no longer on the PlayStation Store. The sad thing is, support for older games, depending on popularity, is either little or none. Now when looking at the game real quick, I see that it came out 5 years ago, so that might be the reason why as to it not being on the PS Store.
However, while also looking, I found on Gamestop's website they have the digital version for purchase for $19.99 (Or Pre Owned for $10). You can try to purchase it digitally there, however, Gamestop's site says that "Download orders are not elegible for returns or credits," so choose this option at your own risk. You can always use their option of Buying it online and picking up in store.
Here's the link to Gamestops Mafia 2 Page: http://www.gamestop.com/ps3/games/mafia-ii/73925