I'm searching for the same rails. I just bought a '96 850 wagon last week and that was the one thing that was missing, no rails or load bars. The Eurorails through volvo list for $399 with the bars for about $150. A good Volvo dealer will discount those prices by 20%. You might also search or call local salvage yards to see if they have any 850 wagons sitting around. I found one place that had the bars and rails for $299 ( No factory warranty and shipping $45 extra). Mounting is easy, each rail attaches w/3 screws. Just be careful when removing the track covers, start at the base of the windshield and work your way back. When reinstalling the covers measure 3 times and cut once. Good Luck.
Scott M.

Thule has them--I think the last time I checked it was about $500 for the rails and mounting hardware. Most Thule outlets will install for free.