Go to a local junkyard and see if they have the wheels. eBay requires shipping etc.

The 15x6 was the standard Volvo alloy throughout the '80's. The 700/900 series typically have a 25mm offset and the Virgo (240 Turbo) wheels have a 20mm offset.

Supposedly Team Dynamics has some lightweight cast wheels in the Volvo RWD offset, but you'll have to check with them on that.