At some poker rooms, the front counter staff will ask how much in chips you want to buy. Most poker games have a minimum and maximum buy-in amount. AR$2/$3 No-Limit Hold'em cash game might feature aR$40 minimum andR$300 maximum buy-in.
They are free, but be sure to tip your waitress, like always. Every poker room I have played there will serve free drinks. Tips will almost ensure timely returns.