Any place you'd buy an invisibility potion, you can buy this. For instance, Kolburn Inglewort in Shadow Haven, in the store on the ledge immediately to your left as you zone in from the Bazaar.
I found that they are sold in the alchemy store next to the bank in Shar Vahl. It is the merchant to the right (female kitty). Sorry I do not remember the name.
How does this stone work exactly? Does it wipe out all magical effects or just the harmful ones or can you choose which effects it nullifies? Also is there a difference between this one and the eggshaped pumice stone?
Good description of the spell can be found at Casters Realm at this link.
http://eq.castersrealm.com/spells/spell.asp?Id=94
I play an enchanter and often wondered the difference between nullify magic and pillage enchantment (enchanters can cast both.) The nullify magic is a lower level spell for an enchanter but I never saw any difference in the effect. If I remember, I will test these spells against disease/poison spells to see if they will remove them.