The TH is only made in 13" and 14" sizes at the moment, hence why you cant find one in 18". I would be VERY surprised if they did produce this wheel in that size, but pleasantly so!

The TH2 is available in 18", but looks COMPLETELY different to the 17" version you have pictured, so it wouldn't just be a case of filling in the holes like on a 17" TH2 - it would require a complete redesign, which is a LOT of work for just 10 sets of wheels....