Is there any vendors which sell 1-axis slide assemblies, steel-duty - 12" x 16" table, with 12" of travel? Something with full-support linear rails, ballscrew, and sealed-off way-covers; with a shaft at the end for me to hookup a pulley / motor.

Myriad of products in this category.

http://www.danahermotion.com/products/product_detail.php?parent_id=971

Keep your eyes open on Ebay they pop up all the time, I got mine for like $60 and it was $1300 brand new from the manufacturer.

I would be looking for full-support steel rails, and possibly a flat steel surface plate. Mainly I sculpt / route aluminum and plastics, and am looking to upgrade my machine to handle steels, eventually.