I bought a $20 playstation card for PW HD and it's nowhere to be found. The only version there is the $30 psp version, which is rediculous. Does this have to do with the PS store update? I know that I've seen it there, and the reason I'm interested in only Peace Walker is because it's the only MG game I haven't played.
As far as I can remember, the Metal Gear Solid HD set has never been infused with the PlayStation Store. You could always buy the original Peace Walker, but then you'd need a PSP to play it on. You could also pick up the collection in its entirety so that you have all the available games.
Though Konami may decide to put Peace Walker on the store somewhere down the line, it's not been the case yet. The "Day 1 Digital" initiative has only been on the move since very recently, whereas the Metal Gear Solid HD Collection is a 2011 release. Based on that information, you might be better off spending money on the whole megillah.