J.G. Pasterjak's DIY ladder dolly a.k.a Cinemover is a solid little track dolly. The adjustable width makes it easy to find a variety of different tracks from straight lumber, square tubing, or round rods. Using heavy duty steel perforated square tubing as the chassis and skate wheels / bearings makes for a smooth ride. [Thanks J.G.] Still a hand made DIY (not mass produced), so quantities are limited, and you can check out details if you want to make one yourself. If you're not into building or buying, maybe winning a contest might be up your alley. There's a few RTG (ready to go) versions and other information available on the eBay listing following the link (click here).

Cinemover Adjustable Width DSLR Video Ladder Dolly