Actually there is an old model of the 1/100 VB-6 Monster, I can't recall the name of the company who produced it, but that particular model is the basis of the 1/100 VB-6 Monster produced by Yamato. This model is uber-rare, and a couple of years ago there was an attempt to recast the model for members here on MW. Do a search for it and you'll get a bit of history on this.
There is also a HWR-MK-II Monster in 1/160, being the biggest model produced by Imai. Usually these go for about $200.00 plus on ebay. Then there is the 1/200 being the next in line.
Good luck.