I'm brand new to bonsai and looking to get started, I'd like to pick up just a simple juniper type. I live in Central Florida (zone 9b) and have decent weather most of the year, but I'm seeing most places say to do the wiring in the winter, and trimming in early spring?

It's now late april and I was planning on driving to some local nurseries, however few there are, to see if I can find a nice plant or two to pick up. I assume it would be way too early to take them out of their nursery pot/soil and move them into some bonsai soil and pot, but what about trimming it down to at least shape it? These juniper stocks seem to basically come in giant flat pads of bushes and everyone trims them down to the rough tree shape they are going for, then wires, but I dont know if it's too late for any stock I would pick up now.

Should I just grab something that I dig around in and find has a nice interesting shape, and then leave it until next spring?

I do not know what plants do in Florida this time of year.

in general ,I prefer to get trees in the right substrate first, and do styling second, using the buld of foliage to re-establish the tree in the pot. Doing this in winter is easier on the plant, because the roots are not actively growing and the water demand from the plant is less high. That being said, I have repotteed mid-summer too. But that is someone who has some experience doing it, which may make all the difference.

Acquiring trees now, leaving them in their pots and figuring out how to care for them before doing major work on them would be a good strategy. There's no point in spending time and effort on a tree that you end up killing due to over or underwatering 2 months later.
Repotting and trimming/styling is stressful to a plant. You want them to be strong before you disrupt them. I generally leave new trees alone for 3mo to a year or more (depending on the species, its age/size and the time of year that I get them) and just let them settle into their new home. If the substrate/soil they are in holds too much water I will repot into something more suitable, generally staying with the same pot or a similar size.
(I have no basis for the following belief, but I think that a lot of tree selling places have too many plants to check on each plant on a regular basis, so whatever conditions/practices work for the masses has to work for the individual tree which may not be ideal and when you bring a plant home it may not be in optimum health.)

Bunsen33 wrote: (I have no basis for the following belief, but I think that a lot of tree selling places have too many plants to check on each plant on a regular basis, so whatever conditions/practices work for the masses has to work for the individual tree which may not be ideal and when you bring a plant home it may not be in optimum health.)

That's my experience too.
Leaving them alone and caring properly, doing things like cleaning it from dead branches and foliage, taking away fallen leaves on the soil, mildly fertilising, preventive light use of insectcide and fungicide (I prefer organics), and watering as much as the species need, all have a good impact in the health of individual plants from nurseries.