If you don't get the abono for the Abierto, the tix will go on sale the week before each game (Monday morning IIRC).
If you're thinking of going to more than 1 game, it's usually easier just to buy the abono and it winds up being cheaper. I think if you do dorrego lateral, it's only 220 pesos for all the games.
If you do only want single tix, just be aware that the semi & final usually sell out so tix can be tough to get - basically, you have to get them the morning they go on sale, otherwise you will probably be SOL.