cr0sh:
You really don't have many options; there are rubber track manufacturers, but they are generally meant for heavy equipment and even the smallest would be too large for your needs (ie, small ride-on excavators and dozers). Another possibility, but still likely too large, are tracks meant for snowmobiles (also called snow machines).
Also snow blower tracks: http://www.build.com/ariens-72600500-track-drive-assembly/p2058533?source=gba_2058533