Login online (on your pc) and purchase it. It should then be available on your download/purchase list via the store on your PS3.
That's how it usually works iirc, but honestly I've only ever added PS+ stuff via the PC and never made any real purchases.
I can't see why it would be any different...but it seems odd that you can't find it on the store via your PS3
Edit:
It does seem odd so if you think it's a risk to try the via PC purchase then just do what you were going to do...get rid of the used game and buy a new copy.
