The cables are most commonly found in 5' 10' 25' 50' 75' 100' lengths. Can be found at computer stores, some larger electrical supply houses, computer shows. Good sources to order online are
http://www.blackbox.com/ and
www.anixter.com Blackbox will also make and test custom lengths.
If carpet is in place, use a utility knife and cut a "T" about 3" and fold the two corners created back, drill the hole, run the cable, fill the hole with some silicone. Carpet will lay back down with just the cable coming up.
Good luck
Chas