New from A2Z is this tidy conversion kit for adapting existing Campagnolo 9 and 10-speed rear hubs to single speed.
Note that it won't give you a "fixie" because your existing freehub body will continue to do its thing but it's a good start for little money and might give you a successful trial that inspires you to invest in a whole new fixed track hub. More likely, you'll be perfectly happy with your single "free" gear.
In the kit you get a 16 and 18-tooth sprocket - choose your weapon - along with 2 x 7.2mm and 1 x 20mm black anodized and laser-etched spacers, the latter to get the chain line correctly aligned. Price is £14.95 from Windwave
This adds to the already-existing kit for converting Shimano 8 and 9-speed hubs.