Some insurance companies can work it in to your existing policy, but it's a bit dependent on age, no claims and so on. Also depends on the company. Smaller specialist insurers will do it as will companies like AdrianFlux and other "performance" insurers or however they like to market themselves.
I never bothered with it, but I don't use my S15 as a track car, I bought a track car specifically for track.
I would only take it out on my track car if I was attending a public track day at a circuit I haven't driven before, much more likely to get hit/hit some numpty.