New: The cheapest you will find is the surface grinder from Harbor Freight. It's about $1000 if I recall, and a mag chuck from them will go for $100 extra.
Used: Look for KO Lee, Boyar-Schultz, or Harig. There are others, to be sure, but these units in 6x12 are plentiful, and have a small footprint if your shop is crammed for space. Expect to pay $1000-$2500 for a solid unit with mag chuck included, however there are plenty of deals to be had.
If you are mostly interested in flattening billets, I suggest converting it to use a 2x72 belt.
Good luck. :thumbup: