Tiffany blue is the green/blue shade associated with the famous jewelry store. The paint color is copyrighted in some states, but mixing a very close approximation is possible. Mixing the exact shade might take a bit of time, but with a little effort, you can get a near exact match.
History
Tiffany blue is the name given to the signature color of Tiffany & Co, the New York jewelry store, popularised by the Audrey Hepburn film, Breakfast at Tiffany's. Although in some American states the shade is copyrighted, the shade does have a Pantone number - PMS1837. The color is not commercially available and cannot be seen in the Pantone swatch book.
Color Matching
Matching a color to a physical object is easiest, so first find something in the shade you want to match to. A Tiffany & Co box would be ideal, but the object could be anything from a ribbon from a wedding favor to a shade in a magazine or from an item of clothing.
Mixing Paint
Tiffany Blue is bright with a fair bit of green in it, so begin by mixing a pale blue paint with green. Do this by adding tiny spoonfuls of green at a time, and mix very well each time. Adjust the brightness by adding white. Again, use tiny spoonfuls of white and mix well. Constantly compare your work to the original sample, and always work in good light.
Paint Supply Shop
If you're looking for a large quantity of paint, a paint supply shop likely can help. Many will color match to a sample, either one you've mixed yourself or a physical sample. So take along your initial inspiration for the color, or a swatch of the color you've mixed, and ask the clerk to match it.