You can use Blooming Purple Moss to lower the Toxin and Poison status bars or wait until you have been affected by either of these to remove the ailment. It is highly recommended to have a few of these assigned to your Hotkey slots before heading into Blighttown.
The Undead Female Merchant will sell you as many as you like for 1,000 Souls each. You can farm all three moss types from the Demonic Foliage in Darkroot Garden. The Blowdart Snipers in Blighttown will often drop them when defeated.
A Sorcerer Class player attacking some Demonic Foliage at the start of Darkroot Garden.