Titleist doesn't offer a service like some of the other OEMs.

Promoting a custom fit, mass merchandising used components don't fit well with the core concept.

Aside from the local retailers in your area that may some sets on the rack, you can check with the larger internet used club sites.  Most of these also have a presence on e-Bay.  3Balls, Global Golf, and Second Swing promote a guarantee of authenticity.  Be sure you know what you want, with the amount of customization, many of the sets will have different lies and shaft lengths than a "stock" Titleist set.  There are other large volume dealers that also will guarantee authenticity.  There are counterfeit clubs out there - the first sure sign is a price too good to be true.  This is neither a recommendation or complete list of dealers to check.  I have used the first 2, not necessarily for Titleist products and if there was a question, they responded quickly. 

I would try Golfgalaxy, Golfsmith, or anther authorized retailers.  They are fairly knowledgable and will be able to spot counterfeits.

Golf Mart or anything of the Roger Dunn nature.  They have a 90 day playability guarantee.

Golfsmith is a bit pricey

Golf Galaxy is cheaper but has a limited selection.

Check if Golf Galaxy / Roger Dunn or Golf Smith's has Demo Day coming up and wait on the trade in to come in.