jaket,
As bmaddox said, 4 Rivers is about the best in the area, though it's a bit of a drive from where you are. On Vine street in Kissimmee, Fat Boy's BBQ is pretty decent, though not outstanding.
For a unique "Old Florida Institution" type of place with really solid BBQ I'd recommend Peebles BBQ in Auburndale, about a 30-40 minute southwest of Kissimmee. It's a family owned and operated place that's been around for over 60 years operating out of what appears to be a reclaimed and converted chicken coop with a dirt and sand parking lot. No frills and very rustic with a limited menu of the best BBQ in central Florida. They're only open Thursday, Friday, and Saturday and it's cash only. Also, they're closed for the summer as they don't have air conditioning, and don't plan on having it installed anytime soon. It's well worth the short trip.
Also, if you're going to be around this coming Friday and Saturday, 3/27-3/28, the Florida Heritage BBQ Classic contest is being held in Kissimmee's Osceola Heritage Park. It's a KCBS sanctioned event, and the State championship. It's free and open to the public with food and drink available.
Enjoy your visit.