Venture on a little further. You've not said enough to let us help, but just try going a little further into the game (NOT into Death Mountain) then you will be able to buy it.
Also, uh, just checking because a friend of mine did this...please tell me the general store you're trying to get into is the one in Kakariko and not the one in Ordon Village...
yeah well here's what the strategy guide says
"when you return to kakariko village,you'll discover that youngster malohas opened his own shop,Malo mart.the mart's topshelf item is a hylian shield.you can buy it elsewhere.but malo's price(200 rupees)is the best deal around.purchase the fire-resistant protector before you head back to death mountain."
there that's like all it says bout it