The system looks like it shares the chassis with the Alienware Aurora R5 -- the fan from it should fit the XPS.
Try calling Spare Parts (1-800-357-3355) to see if they've got one.
I had a similar question. In addition - what way should the front mounted fan blow - should it be pulling air in from the front of the case or pushing air out the front of the case?
the hack job I did on one of our 8700s is in this thread.
once the plastic was back in place it is invisible, and it made a big difference.
Another suggestion. If the heat is coming from a high-end graphics card, remove the extra PCI covers and this will give a nice exit path for the fan.
On the 8700, I used a Y-connector to the fan header running the exhaust fan.
Great! Should help airflow a lot in that case. Were you able to directly fasten the fan to the front grill or did you use some radiator stand type mod?