I was a flyfishing guide in KW for several years. You are going to KW in the second-most difficult month of the year to find a guide (the hardest month being May), simply because the tarpon run is in full-swing. My best recommendation would be to contact a couple of guides who might be able to refer you to someone who had a cancellation. Lenny Leonard was mentioned in a previous post, and he is a very good spin-fishing guide. If you are fly-fishing, he can certainly accommodate. The key is to get referrals from trusted guides and hope that if they can't help you, some of their friends can - again you are hoping for a cancellation. Most of these guys have regular clients who take the same dates or tides every year, for 1-4 weeks at a stretch, from Feb-July.
John O'hearn: 305-304-1730 or 305-872-2862 One of the best, and he has a very large network. Anyone he connects you to, trust that it is a quality guide.
Lindsey Harper: 305-304-1970: Definitely salty, but knows his stuff and a lot of fun. Again, a large network.
What I would caution you against is booking through the Duval Street booking kiosks, or even the local fly shop (Saltwater Angler). It seems like every year more guides show up for 2-3 months of the year, hang out their shingle, and are instant guides. And having no network or local contacts, rely on the local booking services who take up to 40% of the daily rate as a booking fee. Some do not have the requisite licenses, insurance, etc.
Be patient, work the phone and guide network, and you should be able to find a reputable guide. Best of luck!