I've got a florist shop that has me do them a couple of times a month. The vast majority of what they bring me are pre-made plastic runners. The plastic is ridiculously thin, almost like those disposable picnic table cloths. We put a 36" tall vinyl initial on them. I assume the material is so cheap because they're used for 10 minutes, nobody really notices them and then they get trashed.
They've brought us a few that were on nylon runners with decorative fringe. Those, too, got vinyl graphics.
The ones we've completely printed for them because they needed a pattern or larger graphics (including a family crest that had a dead pig on it) we just ran on 10 ounce banner material and left the edges unfinished.
Runners are nothing but another way to needlessly throw money at a wedding....but hey, if they're throwing it your way then they are a needed accessory.
On a side note here's a link to a website that makes custom fabric ones that have non-slip backings and are custom painted. The interesting part to me was that they actually show uses for them after the bondage ceremony.
http://originalrunners.com/after-event.htm