There are many outlets available for coaxial cable. Try Mars Music, Guitar Center, etc. or do what I did, go to Walmart and but a high quality video cable. They are pretty much all the same, with the exception on the Moster Cable stuff, which in my opinion, is overkill unless you've got a $100k studio and have a big budget. Here's a link that includes multiple vendor's and their web sites: http://www.harmony-central.com/Recording/manufact.html#cable
I'd be wary of trying run 20' though. It may cause some problems. Digital signals are weak in nature and you may experience a high BER or synch problems.
Thanks for the tip on the long cabling--I got confirmation on it from another website. I'll buy a short one and move my VS-840 closer to the computer (they're in different rooms). I just might try Walmart before spending $40 on the Monster Cable. Thanks again!
If you're using SP/DIF, it's a standard 75 ohm video cable. Make it short and a good one. What ever you do don't use an audio type jumper just because it has RCA connectors on it.