You can buy one at the bazaar in castle town for 80 rupees (less if you got the discount from the guard). There's also a free shield under a grave in the graveyard. Its in the 1st row of graves you see with flowers in front of it, but you can only open it at night.
But also, you can avoid the rocks at death mountain by backwalking (hold Z and down).
1.- Go to the entrance of Hyrule Town 2.- Enter into the house that is near the entrance 3.- Break all the jars 4.- When you are done, leave, then enter again until you have 80 rupees. 5.- Go to the bazaar (it is in the plaza) 6.- Buy it.