Several very reputable firms advertise in Flight International, Pilot and other aviation magazines.
Its a relatively simple process, but the premiums - considering what you actually get if you had to make a claim, as you may have guessed, are very expensive.
I enquired about 18 months ago for loss of licence insurance ( as an ATCO), and the best quote I could get was £350 per annum for a one off £30K lump sum payment.