65-68 Plymouth Fury, 65-68 Dodge Polara/Monaco and 65-68 Chrysler Newport - all offered as standard equipment a manual steering box and associated hardware.
Of all three lines, the Fury is the most likely to have manual steering - I've seen a lot with it.
Firm Feel, Steer and Gear and a few others offer good quality remanufactured steering boxes, but keep in mind you get what you pay for.
Respectfully, why would you want to go to a manual setup? The power steering in C bodies is somewhat of a necessity at lower speeds, especially if you get an upgraded box. I'm interested in your reasoning around the swap.
Same for manual to power brakes - while some love manual brakes, typically most enjoy power assist on our big cars. Upgrading to power discs is easy and very effective improvements are gained using all factory parts.
eBay has lots to offer here:
Manual Steering box search:
C body manual steering box | eBay
Manual steering C body pitman arm
NOS MOPAR 1965-6 C BODY MANUAL STEERING PITMAN ARM | eBay
C body Idler arm (I do not know if this is compatible for manual steering applications)
NOS MOPAR 1965-73 C BODY STEERING LINKAGE IDLER ARM | eBay