Ebay is your best bet, these went out of production about 2 years ago when MDC was sold. Since then they have been scarce. San Val had a good supply of them but when the owner retired earlier this year that well dried up.
It seams the MDC G guage molds were either sold to or are being used under license by Piko for their entry into the US market to provide freight cars for their new 0-6-0T, so its possible we may see a reissue sometime next year of the Big Hustler under the Piko nameplate.