Well, in Dearborn MI I would definitely invest in some winter wheels and snow tires. The Tethys wouldn't be bad, but the tires would be more expensive in such a low profile.
Back to what you are looking for, check Tire Rack
http://www.tirerack.com as they're one of the larger aftermarket wheel companies in the US. They probably have a good selection for you.
Also, you probably receive the Volvo magazine that is sent to all new Volvo buyers. Volvo is offering a $200 rebate on accessory alloy wheels in the current issue. I think they make 2 different 18" styles that are factory wheels.
Last, if you want the shiny look, consider machined or polished wheels. The chroming process makes wheels brittle and more likely to crack. With 18" wheels and being in MI, I wouldn't by liking those odds.