I have been researching this costume lately as well (this is going to be next years project), unfortunately I have found very few resources for it.
Most of the hard parts can be found on thingiverse.com, search for the Sith Stalker armor (many of the hard parts are the same for both costumes) and you'll also need a couple Tusken spikes as well. I was planning to either 3D print the trophy sabers or make them out of PVC since they don't need to be anything more than belt hangers. As far as the robes and other soft parts go, the tunic and pants are pretty much the same thing that Jedi wear so you'd just need to someone that sells those items (check etsy) in the colors you need (Tan tunic, Brown pants). Unfortunately you're probably on your own for the skirt and neck scarf unless the person making your tunic and pants would be willing to make the additional parts, but that's something you'll have to discuss with them
Hopefully this gets you started, but if you have any other questions, lemme know and I'll do what I can to help.
If you haven't already checked it out, here is the CRL for the costume, so you can double check as you build:
http://databank.501st.com/databank/Costuming:SL_starkiller_desert