Found this on eBay. Right kind of engine but it is Nickle Plate Road, not Polar Express. Current eBay bid is $44. Looks like a good seller.
The engine in the movie is a 2-8-4 Berkshire painted black. In the book, the engine and tender are solid black. In the movie, the tender had "POLAR EXPRESS" on the tender in white letters.
There are a number of similar locos on sale on eBay with current bids below $50. Pretty much any engine with 8 drive wheels and both a leading and trailing truck would look good I think. A little black paint to hide the prototype livery and I think you'd be in business.