If you missed the original Killzone way back in the prehistoric year of 2004, now you have two options: either buy Killzone Trilogy and get all three Killzone games, or you can download the original Killzone as a standalone download from PSN for $14.99.
Killzone HD will, of course, be PSN-exclusive, and you can go ahead and buy it on October 23rd. At least you don’t have to leave the comfort of your home to buy it in physical form. Who still does that, anyway?
Source: PlayStation Blog