For about $200 I found on eBay a full set of NEW Nissan Frontier wheels in 15" and put 30X9.50 rubber on them. The only issue, and it's a minor one, is that you need to relieve the inner lip of the wheel about 3mm for A-arm clearance. Running a fine-grit sanding drum chucked in a hand drill around the center bores will help too as they can be just a little tight.
So take your time and look around, there are some deals to be had on 16" and 15" wheels. Just be sure to check the center bores on non-Isuzu wheels.
If you'd be interested in selling just one of your 18" wheels let me know, especially if you've got one in poorer cosmetic condition than the others.