Dave,
Mart Aviation are one of the many companies that sell ex-raf helmets but you'll need deep pockets, they cost about £400 or more! You occasionally see them on ebay too, for similar money.
You can get the adapter for them at most pilot shops. I know for a fact that Transair sells them.
Just a point to note, Tornado helmets use different connections to the other types, and you can't get adaptors (as far as I know.)
If you're buying to use in an aerobatic aircraft then I'd recommend a later Mk 10 type helmet, as the Mk 4 is a little heavier and you can hurt your neck if you're not used to the load.
Hope this helps.
Blunty