rope on scabbard?

You guys will have to forgive my ignorance, but I was wondering what the rope I always see tied onto the scabbard (or saya?) is called, and what is its use and/or significance. Also, can you buy them?
Thanks again.

It's a sageo. It's used to secure the saya to the obi (belt/waistband).

You can buy them off the web - try Tozando crash - you can get reasonable ones through ebay too every now and again too.

Is there any significance to different colors/sizes/types?

There is a specific way of fastening it to your belt too, it just not an old granny knot. There is a certain way it lays next to your scabard when the katana is on the floor also.

Within some styles there are restrictions on what colors are appropriate, but you'll have to ask your instructor about that. It varies from style to style.