I am doubtful they will have a gas piston in stock, but Al Simmons in Hamilton is also a Magnum Research dealer, so they should be able to order it for you.
However, I recently required a new safety lever mounting screw for my old DE.
I was about to call Simmons, but thought I'd try contacting Magnum Research myself to see if they could ship the part directly to me.
I went on line and obtained the part number I required.
http://www.magnumresearch.com/docs/manuals/DESERT EAGLE.pdf
I called the Magnum Research repair number to confirm the part number was correct for my model, the fellow said I had the correct part number, he asked me if I wanted to purchase the part. I told him I lived in Canada and it was tough to obtain parts for most US made guns here, especially DE's. The fellow said that was not a problem, he took my MC number, mailing address and a screw set arrived in the mail the next week!! Screw set and shipping came to just over $20.00
Sure beat messing around with a gun store as a 'middle man'.
You may want to give them a try on Monday (they are probably closed today) and give it a shot.
http://www.magnumresearch.com/serviceRepair.asp
Good luck.