Part 138466 - £1.95 from Canley Classics for the steering wheel nut
and part 21H6210 - £2.20 for the wheel nut

Here's a quick run-down of the Spitfire parts suppliers I use (that I can remember off the top of my head).

Moss Europe - almost every part, top quality but also most expensive.
Rimmers - second-best parts cover, top quality and almost as expensive as Moss.
James Paddock - only good parts coverage but top quality and noticeably cheaper than Moss/Rimmers.
Canley Classics - quite good parts cover, one of the cheapest but not always 100% on quality.

These are just my usual suspects and there are others out there and based on my personal experience.