A lot of the civvy jet warbirds aren't really up and going. Several of the T-33's and F-86's are owned by people that later decided they didn't want to afford keeping them annualed and airworthy and are really static displays. In many cases the owners would sell immediately if they got an offer in the ballpark of what they paid for them.
Most of the imported jets like the Migs, L-29's , Iskra, Fougas. Hispano Saetta's, Saabs, Jet Provosts,Strikemasters, etc. were imported based on someone speculating in the market of jet warbirds. They were bought as investment units to be resold for a profit and if the market hasn't fulfilled their expectations, then they quit having them uncrated, assembled and gotten licensed and flying. They're just sitting around the country and the owners would kiss you on the lips if you made them an offer to help them recoup a little of their capital.
Lastly, there have been a few aircraft in civilian hands specifically to fill some govt. contract like radar penetration exercises, Test Pilot's School, etc. A few I remember include the English Electric Canberra(B-57), and the Fairey Gannet in the U.S. And NO, no one can afford to operate a B-57 just for fun. They didn't get rich, by being stupid with their millions.

millions.

That kinda reminds me of the Victor or Vulcan bomber a civilian group owns in Britain. They maintain this bomber to airworthy condition, but the British government will not allow them to fly it because its considered a nuclear bomber. Last I heard they do taxi it around the field once on month and thats it. The last time I saw a Avro Vulcan bomber fly was at the 1978 Dayton Air Fair in Vandalia, Ohio. Man was that a site to see! One British millionare owns a Blackburn Buccaneer S.2. The British government would not grant him a permit to fly it in England so he moved to South Africa and flies it regularily down there.