Here's a couple options you may have*:
1998 Volkswagon Passat Wagon TDI (32/45 diesel) . . .though I've heard 60 mpg.
1994 Plymouth Colt Vista: 21/26 w/1.8L and 5-speed
1994 Ford Taurus Wagon: 18/27
1994 Ford Escort Wagon: 26/34
1994 Toyota Corolla Wagon: 23/31
1994 Honda Accord Wagon: 22/28
1994 Saturn SW: 24/34
1994 Chevy Caprice Wagon: 15/23 ok, maybe not
*All 2008 EPA numbers
If you want do do some of your own further research, go here for mpg ratings:
http://www.fueleconomy.gov/feg/findacar.htm
And here for interior volume and other specifications:
http://www.edmunds.com/apps/usedmatrix/UsedMatrix