Hi Kendel - I went through sourcing these two parts about 1 year ago when I found a crack in my spinner and backplate. I can tell you it was quite a challenge. New from Cessna the spinner was about $1300 and backplate was $1000 (so $2300 total for new parts). No one else sells new PMA parts. I contacted every salvage yard I could, and they all laughed when I asked about the aft bulkhead - they get multiple calls every week as this part cracks frequently and replacements are so expensive. I forget if any had the spinner.
After a couple of weeks of searching, I was able to get a new-old stock spinner for about $300. For the bulkhead, there is a guy who makes replacements under the "owner produced parts" rule. He made a beautiful spinner and charged about $400 I think. If you want to go that route I can get you his info - he worked very quickly and made a beautiful part. I did check with my mechanic before doing so to make sure they were okay with going that route. He is an A&P that works for the FSDO so when he said it was okay I felt good about it.
Unfortunately with the L model, I don't think the skull cap is an option.
Good luck with your search and let me know if I can help with any more info.
