Obvious question is why? They aren't toys or status symbols and I can't think of any reason you would need one on a bike in this country. In the middle of the Arabian deserts where you are - that's a differnent matter.
However, if you want one goto any pilot supplies shop. I would recommend Harry Mendelssohn in Edinburgh - www.gps.co.uk - look under Survival Equipment
Nice company to do business with. I bought 4 headsets and a GPS from them a few years ago.
Exactly Mike, the Middle of the Desert. Off road riding is fun, but you or your buddy have a crash and you need help pretty fast. No guarantee the mobile will work dune side