Didn't the loco come with extra spare tires in a little baggies in the box? If not, then I also recommend contacting Athearn. Their customer service people do respond to emails.
If that fails, then a 3rd party vendor (like Chinapig mentioned) can be used.
Mike: In my experience Bog Boy is not that prone to thrown tires due to it being pushed. Kato 4-8-4 locos are much more delicate and can be easily messed up by being pushed along the track. Either way, when I run my trains on NTRAK layouts I usually follow them around the layout. Wireless throttles make this easy.