Some of the tracks like Hard Rock and Reddick allow practice on the weekends. You probably want to call them first and see when there available. Gatorback has a really good vintage track, but I'm not sure they allow people to use it for practice.
The FL Vintage MX club is a great bunch of guys and you might wanna check out one of their races. Also there is some trailriding allowed in the Ocala National Forest. I haven't been over there yet, but hope to make it one of these days.