I'd vote for NAPA, there are 4 or 5 times as many NAPA stores as there are Walmarts. Ed.
According to Answers.com, Walmart has 6,775 stores in the US and Napa (Owned by Genuine Parts Company, Inc.) has 6000. Either way, you shouldn't be far from either one.I chose Walmart because of not only the numbers, but ease of access. All Walmarts have a large parking lot where a bus can park. Not all Napa stores have a place big enough and I don't want to carry a battery any farther than I have to.Either is a good choice, just make sure you get batteries that will fit your needs.Dallas GO BUSSING!