Have you tried a soft or hard reset? (Note that a hard reset will delete all data and installed programs.) If you have tried those already, touchscreens are available at:
PDASmart - Touch Screen / Digitizer for Axim X5
And repairs at:
Pocket PC Techs - Repairs for Dell Axim X5 Touchscreen Repair Service
The prices are rather high; if you can get your hands on an old non-working X5 (i.e., via Ebay, etc.) just for the parts, it might be a cheaper solution.