I bought plain, white twisted nylon from a hardware store that was 1/4 inch thick. The first pack I bought was fifty feet, and I cut it into one thirty foot length and two ten foot lengths. The longer lengths was for body ties like chest harnesses, and the shorter ones for hands and feet. At that time I was tying simple things like spread-eagles and hands behind the back. Price was also a consideration, and I built up more rope over time.
I’ve since switched to natural fiber like hemp and jute, because it has more tooth and the knots and wraps hold better. I use thirty foot lengths, because I’ve also started tying more Japanese-style bondage. It’s the combination of rope and technique that I prefer, but I still have some of the nylon in case the person I’m tying doesn’t like the feel of hemp or has allergies.
I’ll agree that Two Knotty Boy’s videos and books are a good place to get starting ideas, and that safety is important too.
Hope this helps. Good luck and have fun!