There is a limited choice for 18 inch rims but Bridgestone do a tyre in that size but it is more sport/touring, the web will give you a better idea.

For future the acquisition of a 17 inch rear wheel would be the way to get a far greater choice of tyres, there are a few wheels usually for sale on the Yahoo Japan auction site and often go for reasonable money, you just have to judge by the photographs the condition as that varies quite a bit, then shipping charges of course, I had mine sent by SAL 2-3 weeks delivery but cheaper.

My rear wheel cost £60 + about the same for shipping etc., so a good wheel for a relatively small amount.

Other than the Bridgestone I'm not sure if there is much choice other members will help to advise you.

Cheers, Dux.