Not gonna pretend I made this, but here it is courtesy of Tiamat from the official Torchlight forums.
The potions do cost 3000g though.
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.