Many electrical supply houses offer scrap chunks of conduit free, or a minimal charge.This is another reason why I prefer my panels accessible in an unfinished part of the basement and not in a finished wall. I can actually work on it!
I put my critical loads panel near my main, with a 2" EMT conduit running between them. I'm leaving all the existing circuit grounds and neutrals where they are in the main, and simply moving the hots and breakers of the circuits I want through the EMT to the critical loads panel.
It kinda sucked having to buy a 10' length of 2" conduit so I could cut off 18 inches of it, but ya know...