Not true any longer, all the GT3 slcik tyres are now 18" - Pirelli, Dunlop, Michelin, even Hankook are making soft, medium and hard in 18". You would need to purchase from a speciality motorsport tyre company I guess, as these are full race slicks, and not "R-Spec stickies like R888, A048, A050, RE55S, DZ03G etc

For instance, my supplier can do:

Dia Size Tread Tyre Dia Wheel Size
18 210/650R18 210 649 8.0 - 9.0
18 230/640R18 228 641 8.5 - 9.5
18 250/640R18 240 641 9.0 - 10
18 260/655R18 260 663 10 - 12
18 265/650R18 260 650 9.5 - 10.5
18 280/655R18 280 663 11 - 13
18 290/675R18 290 675 11 - 13
18 310/655R18 310 663 11.0-13.0
18 330/710R18 330 710 13 - 14

I'd be using 18" on a full-race car ..................