Local hardware store is a good place to get bondage-friendly rope (avoid kink or sex shops if possible). Nylon works best to start, 7/16" is a good starting thickness. Wash it in a delicates bag so it's a little softer. A variety of sizes is good - you can cut one 20' length into two 10's, get another 20 or two, and a 30-50 piece and you'll be set for a lot of basic situations.

But 50 shades is terrible and lame. Bdsm should be much more healthy and fun.