Hello and welcome to NLG.
The brightness is likely in the back of the CRT. Power off, pull (CAREFULLY) out the CRT. You should see the brightness control on the back. BE CAREFUL what you touch. Even with power off, detached from the slot, you can still get a nice shock from the tube. Just don't handle mess with the big wire that goes in the top of the tube. Mark the position on the control with white out, or tape. Then move it about a quarter of half inch one way. Re-install and see if it's brighter or dimmer. If dimmer, then reverse your procedure.
As for parts, contact Alan at K-LAR. His information is on our home page... you'll see the banner. OR look up NLG member "badbaud." That's him. He may have the parts.