Bolt pattern not important here. Also shouldn't need to use a specialty supplier. Any decent parts store or brake relining shop can supply the parts. If you have specific lining needs, then Porterfield or Rochester Brake and Clutch would be two nationally known places to talk with.
Some incorrect information getting tossed around here.
The original '68 shoes were used for 1 year only. They use the same hardware as used in '67 and earlier. This is true across the line, not just for the 10 x 1.75" brakes. The main difference was using a tab on the shoe's frame.
As mentioned by someone else, in 1969 all the drum brakes started using the bendix II style self adjusters. These look different and rotate the star adjuster in the opposite direction than the early system.
The shoes for the 10 x 1.75" drums up through Feb 1967 were FMS 256. Although not recommended by Chrysler, aftermarket suppliers often interchange them with FMS 151 shoes (used on Fords). Was Chrysler just protecting their exclusive? I dunno. very close match.
March 1968 through the end of the model year, FMS 288 shoes were used. Often earlier, and sometimes later shoes are interchanged.
For the 1969 model year, the 331 shoes are correct and must be used with the new style self adjusters.
The metal frame or backing plate of the shoes will have the FMS number stamped or printed on them.
ref. 1967 Plymouth Service Manual, 1968 Plymouth Service Manual
Master Technicians Service Confernce booklets
68-1 Sixty-Eight Service Features
69-1 Sixty-Nine Serving Highlights
69-7 Drum Brake Service Highlights
http://www.imperialclub.org/Repair/Lit/Master/index.htm
FMS or FMSI stands for Friction Material Standards Institute