Walking through a home store, spotted a stack of glossy 24x24 ceramic tiles. Ran over to grab a steel yardstick, checked corner to corner, edge to edge, this sucker was FLAT! $7.00 ! Glued it to a 3/4" piece of MDF, got myself a shop surface plate for $10.00!!!
Maybe not lab grade, but a good shop grade. Then blew a wad on blocks, squares and gauges.
Cheers!