Koni are the only company who make inserts for the GTiR and these are fairly hard to get hold of. For the price of the Koni's, I'd keep my eye out for a set of coilovers.
A full set of Koni inserts comes in at around £480 iirc. Look around and you can get a set of D2/K-Sport/G-Force coilovers for just over £500.