Check around at local salvage yards, I can bet that most 6 speed cars aren't exactly babied and low mileage parts might be relatively easy to find. The last place you want to go is the dealer for parts! I looked at ordering a 3.9 sedan with the 6 speed instead of my 3.5 GT, but the dealer wanted $12K more! So, yeah.
Rockauto (as you mentioned) is known to carry quite a bit of more-rare parts. Summit Racing or Jegs might carry what you seek as Jegs primarily carries GM performance parts. Ebay would be the last recourse before seeking out dealer parts.
'11 Ram SXT 4x4 quad cab - 5.7L V8; 128,200kms
'09 Malibu 2Lt 3.6L - 140,000kms purchased 1 March 2019
'06 Pontiac G6 GT - 3.5L V6; 162,000kms (rear ended 15 Feb 2019, and given totalled title by insurance)