Go to your local outdoorshop that carries climbing gear. If they have a good selection they will almost always have a large spool of Bluewater II static climbing rope that you can buy by the foot. They will also usually have an assortment of webbing and smaller diameter ropes you can buy by the foot as well.
There are different types of climbing rope that generally fall into two different categories--static and dynamic ropes. The expensive ropes are usually the dynamic ropes and they are not what we need for our purposes. A dynamic rope is designed to stretch (20% or more!) under a load and therefore prevent injury to a climber who suffers a fall of 10-30 feet or more by softening the impact of the rope when it stops the fall. A static rope like Bluewater II will not stretch under a load (or not appreciably--all ropes stretch under loads).
A static rope is fine for our purposes because if we do what we should in terms of adjusting our harness length and move the system up the tree with us as we climb into a stand we should never fall more than 2-3 feet anyhow. Also, a dynamic line will stretch with your weight in it making self-recovery into the stand harder if not impossible.
Bluewater II is the industry standard static climbing and rappel rope. The 11mm (7/16") diameter Bluewater II has a strength of 10,000 pounds. (yes I typed that correctly). As I said, most any climbing shop will have it on a big spool and will sell it cut to length by the foot. I have personally used this rope to rappel off 400 foot cliffs (and shorter) and have trusted my life completely to this rope. I would never buy a rope from Home Depot/Lowes etc and trust my life to it. Bluewater II is cheap enough and easy enough to find that there's no reason not to use it.
An alternative that you can often find is the braided green Army rappel ropes you can buy in surplus stores etc. Its usually sold in 120 foot lengths and I think I paid $48 bucks for the length I bought a few years ago. Here's a link and its like $78 for 120 feet.
http://www.uscav.com/Productinfo.aspx?productid=9492&tabid=548 Keep in mind this is a dynamic rope and its going to stretch when weight is applied so I don't think its as good as a static rope for treestand safety. But it will work and I have personally rappeled out of helicopters and off cliffs using this stuff. Buy it from a reputable surplus dealer and it will be fine as well.
I highly recommend you go down to the local climbing shop and talk to them. If you want equipment you can trust implicitly there is no substitute and frankly its not that expensive.
Here's a place that sells Bluewater II rope online for .84/ft cut to length.
http://www.backcountrygear.com/climbing/ropes-cordage/bluewater-ii-plus.html