If you can't find the iGo at Radio Shack, CompUSA has (available for online orders, not in-store) refurbished
Targus power supplies with multiple tips for $19.99. They handle 90 watts continuous, 130 watts peak.
I'm using the Kensington, so can't say whether the Targus is good or not. There's also a link to a compatibility list in the product description.