Anything that will be used around a hamster should be safe,including cage lids.Kiln dried or untreated pine should be fairly easy to get,which would work nicely!
Like amity said, untreated, kiln dried pine would be your best bet! This is available at just about any DIY type stores. You can also use kiln dried, untreated poplar if you can't find any pine (which is highly unlikely)