Where can I get props for indoor drumline -i.e., backdrops, decorative tarps?

You can check at www.theguardroom.com they basically buy and sell everything from costumes to tarps and props for the activity.

Also if you're looking to get a new floor try:
http://www.colorguardfloors.com/
great prices and a fairly good selection of products, you can buy a blank floor and paint it yourself or you can have a design professionally printed onto it (more expensive)

Most groups I know of make their own backdrops from scratch. I've seen PVC pipe frames with fabric stretched across them, some groups use wood and fabric, or paint foam core board. Many different options out there, all depending on the shape and size you want, however I dont know of any ";off the shelf"; places that sell them.

What kind of backdrops are you looking for?

My indoor line plans to do a warrior themed drill for our next performance and I am just looking for simple staging items.