There are several sites with spoke length calculators http://lenni.info/edd/ being one with a pretty complete array of pre-specified rims and hubs. The DT Swiss site or others (UBI) will give you the option to manually plug in the data if your hub/rim do not show in the linked site.
You'll need the following data:
Rim ERD (diameter of rim measured from screwdriver slot of nipple to same spot on the other side of the rim) - this info should be available or your LBS can help measure it.
Hub Flange Diameter - info should be available in hub manufaturer tech data or on spoke calc site)
Hub flange distance from center (right and left) - same source as above
Spoke cross pattern
Some of the sites additionally ask for spoke hole diameter and nipple length, but the default values are typical and should return a reliable answer. Your LBS should be able to help you validate your calcs if you are unsure.