Not gonna pretend I made this, but here it is courtesy of Tiamat from the official Torchlight forums.
The potions do cost 3000g though.
Quote from Tiamat »
Respec potion mod, allows you to buy respec potions from the general inventory vendor (the one with identify scrolls)
The respec mod causes the general inventory vendor (hp/mp potions, identify/town portal scrolls) to offer the respec potion.
Unzip the structure into your %appdata%\Runic Games\Torchlight\mods\ folder.
You can find this by following the C:\Program Files\Runic Games\Torchlight\Application Data link.
It should look like C:\Users\YourName\AppData\Roaming\runic games\torchlight\mods\RespecMod if you are running Vista.
NOTE: If your vendor isn't selling the potion for an old character, you may have to get his current store to restock his wares. Vendors will restock if you spend about a half minute in town, leave, and return.