To get powder kegs whenever, you just fight Goht (or whatever his name is) again, and make it spring. Next, go to the Goron village and find a small cubby hole in the wall. (I love that name, cubby hole!) Go down and speak to the giant goron as Goron Link and he'll ask you if you want to test to carry Powder Kegs. Say yes, and he'll drop a powder keg in front of you. Throw the powder keg up the large ramps and carefully make your way to a giant boulder up some steps next to the cutscene path to Snowhead village place, where the blacksmith is. If you can manage, throw the powder keg in front of the boulder, thus blowing it up, and then you can participate in the goron races. You'll also then be able to buy powder kegs whenever from the goron in the bomb shop in Clock Town for only 50 rupees. Hope this helps!