Saw that Holly Halston was available in Florida, and I wanted to know if it was legit. I've never booked through an agency before, so any help would be appreciated.

Without more facts, it's hard to say.  

Holly is definitely available through a number of different agencies.  There are reviews here, linked below.  So she is definitely a provider.  Whether or not the ad you saw is legit, no one can say without seeing the ad, or you naming the agency.