What mortar should be used for flagstone?
First of all, a caveat: 


  We only use this dry grouting method with freshly buttered joints, where find that there is sufficient moisture from the 'proper' mortar to initiate setting of the dry mix. Dry mix into dry joints rarely, if ever, produces a good joint 


From http://www.pavingexpert.com/pointing.htm#dry

A basic dry mix is 4:1 sand to cement.  That's by volume, not weight.  I'd try to find a fine sand, and preferably sharp cornered.  

You may be able to find a premix at your patio/garden center.

The problem here is that all cement compounds require a significant amount of water to cure, and your joints will need water from the bottom as well as the top.  You might be better off using a wet mix and just taking the time to apply it carefully.