There are a few options
1: Scratchbuild
Probably the hardest to do so only try if you know what you do.

You'd need plasticard and plasticard rods and a lot of time. I've done something similar for my scratchbuild fabius bile.
2. Convert
Use already existing bits but convert them up into servo arms. Ive seen necron legs used as the arms and some techy looking bitz attached to them. Not my example:
3. Existing arms
Like already linked there are a few 3rd party producers that make servo arms. Maybe look out for puppetswar or chapterhouse studios. They might have some for you.