As in, how does one go about it?

I don't mean the replacement limbs (which assume you got soething hacked off), but stuff like the MUI, the Auguer Array, or the Servo-Harness?

You can get the servo-harness on a per-mission basis by paying the requisition everytime (as it is removable, just like the servo-arm), but what about the non-removable ones, like the aforementioned Auguer and MUI?

Techmarines get them automatically as they cross from Renown to Respected, and they start with Servo-Arm and one other Common craftsmanship cybernetic. There is a table 5-20 on page 178 that lists the requisiton and renown costs for the "normal" acquisition of cybernetics.

EDIT: apologies, I didn't read the question closely enough. On page 177 however, it states that the Requisition cost for a Cybernetic is a on-time payment, once the cybernetic is installed, the character possesses it until he dies.