I’ve purchased car batteries and motorcycle batteries from both Walmart and Advance Auto Parts, and the car batteries from both sellers tend to last about four years in the Florida heat.
The problem with buying a battery from a mechanic is that he or she will charge you labor to install it, while some auto parts stores will install it for free. I prefer to install my own batteries, partly because I don’t want to pay for the labor, and partly because I don’t trust a non-mechanic auto parts store employee to do the job.