IMHO best to wait until you're over there as the choice and cost is far better than you can pick up this side of the pond. We bought ours from the bookshop next to Eckerd in the Florida Mall. They had quite a choice and ours only cost about $4 and covers Sanford down to Kissimmee and WDW.
I have bought quite a few of these over the years. The online shop is pretty good. I like to have the maps beforehand so I can familiarise myself with the region. Not so important for Orlando, but head out to the midwest and the signage disappears!
I got some really good maps from Waterstones (albiet the 'megastore' on Manchester Deansgate) & then forgot to take all 4 of 'em! Picked up some nearly as good in the first Walmart I saw.
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