Although a lot of people suggest places like Sky and Flux it is still worth a try going for the normal places too. I have always had my S15 insured with Admiral as they offered the best price.
Admiral are worth a try, I had my first RX7 at 21 with them, for about £800. That was about 5 years ago. They allow some modifications, just not huge power increases.
I use Brentacre for my Turbo, very reasonable and no increase for mods that don't affect the power. IIRC they have power 'bands' that - if tripped - increase your premium.