Your 1909 has the same screw spacing as other large ring Mauser 98s.
You can buy what you are looking for from Recknagle at NECG for well over $1000 or from the following US firms. Sunny hill ,Blackburn ( Swift Bullet Company) and Dwayne Weibe all offer drop box mags that will allow 5 magnum cartridges for about $500.
http://www.sunny-hill.com/2008PriceList.pdf
http://www.swiftbullets.com/swift-blackburn-gun-metal-s/1818.htm
http://www.customgunandrifle.com/products/bottommetal/mauser-bottommetal