Antipoison potions can be purchased at Jiminua's Jungle Store on Karamja or purchased from players. There is also a spawn point in the Observatory Dungeon in a chest north-east of the stairs leading up to the Observatory.
It should be noted that Antidote++ cures poison, provides immunity to poison for 12 minutes, and is also the cheapest alternative.
As of an update on 3 May 2018, antipoison has been made accessible in free-to-play worlds. Players may trade in a cadava berry, a limpwurt root and 5 coins at the Apothecary in exchange for an antipoison, and free players may now purchase antipoison at the Grand Exchange.