There are several companies that make walkers of appropriate scale. Dust is the most common, but there are others such as gears and guts who do 28mm versions of Gear Krieg mecha.
http://www.adpublishing.de/html/gears_and_guts_.html. for sale here:
http://www.brigadegames.com/Gears-and-Guts-Weird-WW2_c_384.html
Unfortunately, most of them are a bit more heavily armed and armored or larger in bulk than an
IG sentinel.
The end result is that they will look great on the table with
IG, but you'd have to find different stats for them.
All that said, this one from gears and guts might be an ok sentinel proxy if you don't mind it being a bit bigger and harder to hide.