Walmart - but don't buy the cheapest one. The higher end Interstate will probably last about the same as any other.
I just had to do the switch from Die Hard because their local store is closing. Research led me to the Interstate about as much bang for the buck as any other.
Another alternative is to go to the dealership parts counter. I'm starting to buy more and more of vehicle maintenance items from them because not only do they have it in stock, but the price is comparable if not slightly less once you get to know the parts counter folks.