When I search for wheels by size I like to use ebay if only initially to see what's out there. Just type in "wheels" and use the search options on the left to fine tune your parameters.
I use eBay a lot for wheel searches. The thing I'll add here is that not all the sellers enter the wheel specs in a way that the little check box parameters pick up, so sometimes using those check boxes on the side will eliminate wheels that might actually work. It's a pretty good way to do it most of the time though. Occasionally I'll just search for something like "5x114.3 18x10" which will show pretty much all the offsets then scroll through quickly to see if there's something I like.
Summit racing also has a pretty good search that will let you check multiple width and offset options. They have a decent selection too, but obviously they don't carry everything.
The other thing is that most of the manufacturers have decent websites, so if there's a style you like you can go to the website and see what size/offset options they have. Sometimes you can special order stuff that won't show up on eBay. So if you have a favorite wheel manufacturer that can work pretty well. I also follow a couple wheel shops on my eBay profile, so when they get new stuff it shows up in my feed. :D
