First off...
before I go, I always make a plan of where I want to go, and call the nurseries ahead to see if they have what I want. If not, I'll find other nurseries/collectors to be certain I'll get the plants I want. I always try to coincide my trips when the most fruit is ripe. So I try to go to FL during the summer months to try the most fruit. It's not the wisest time to go during winter to FL for plant shopping as the trees can get shocked when they go into the greenhouse depending on what you get. If the nursery I called has the plant, there is a 99% chance that it will be there and looks good. All the nurseries I've visited in FL have never let me down and the plant quality has always been great. Nursery inventories always fluctuate depending on what is sold and there is never a particular season when you can be sure they'll have what you want. That said, the best time to go is whenever you have time.