In simple words.... Google is your best friend !
About Encoders.... In B737 you need 4 Styls.... Two are easy.... the other two are problematic.
1. Single Encoder ( Like the CRS Selector on MCP) ....
Simple google for "Encoder" Thousends of hits.
2. Dual Encoder ( Like COM1 Selector) So a Encoder/Encoder Build.
Also Available.... Whatever a bit more "rare" and much more expensive ..... Google for "Dual Concentric Encoder"
3. Tripple Encoder (Like ADF Selector) (Problematic) Maby you found it in special Cockpitbuilder stores. Pretty sure verry expensive.
4. Encoder/Switch Combination ( Like Efis, Heading/Angle , ) As i know not directly available.....
Alternate is DIY or Prebuild Mechanics.
Simply google for "Do it Youreself" or so.
Basicly you can build every special system ( Nr 2,3,4) with a mechanical solution (Mostly Cogwheels) ..... For a Dual encoder this is also possible but not profitable cause the mechanic cost more money/time as ordering simply a premade Dual Encoder.....
For 3 and 4 its different. Here you not find premade solutions or they are extrem expensive..... So you must build it youreself or buy a mechanic for it.