Eastwood has a tool that uses a MIG welder to weld a stud to the sheet metal. They also sell a puller attachment that grips the stud.

Every stud welder I have seen requires the studs. I have never seen one that does not require a supply of studs.

One other idea in the same vein is a trick with copper brazing rod and pennies. Braze the penny to the end of the rod. Polish the penny so it is smooth. Coat the polished penny with lead solder. Then sweat solder the brazing rod and penny combination to the sheet metal dent. The brazing will withstand the lower heat of the sweat soldering. Then pull the dent out using the brazing rod. This has the advantage of being able to reuse the rod / penny combination again and again. Simply heat enough to release the sweat soldering bond.