You should be able to get 1/4" x 1/4" TRS cable in any length your little heart desires from just about any music store (Sam Ash, Guitar Center) or any online music store such as Zzounds, or Bay Audio, Mars Music, etc. Then just get a 1/4" x 1/8" adapter to go into your sound card.
As long as you use balanced cable the longer run should not present any problem.
(Mixsit - first time I've ever heard of losing highs through cable runs over 15' Generally noise is the only problem I'm aware of in long runs, and balanced cables should take care of that. You may want to recheck your info.)