Maybe a robotics specialty house?
I put together a B.O.M. for a design I sketched up (it’s pretty obvious, and I lost the sketch) here: Shapeoko CNC Router, Rigid, Accurate, Reliable, and Affordable
Mounting Plate for Nema17 or Nema23 — use a pair to mount the motor at the top of the plate: Mounting bracket for stepper motors - RobotDigg
ACME Threaded Z-axis Nema23 Stepper — mounting the nut looks as if it will need a custom part ACME lead screw threaded Nema23 Stepper - RobotDigg
KP08 Pillow Block — would probably need to cut a slot in the bottom of the plate KP08 Pillow Block for 8mm shaft - RobotDigg
I’d definitely try @cgallery’s idea of a counterweight first though — negative of course is doubling the mass involved, but I think the motors would have the torque to cope w/ it.