In reply to Dingerbell:
Another option:
http://www.gooutdoors.co.uk/beal-karma-9-8-30m-single-rope-p390862
Benefits being, it's a climbing rope with a good diameter, but pretty cheap... I got it a bit cheaper that £39 though...
I've used this quite a few times on scrambles, as the short length means it's light, whilst the chunkier diameter makes it more durable and handle better. Plus, you can take a lead fall on to it, if you're doing a harder scramble and decide a rope is necessary... E.g. On a grade three scramble like Pike O'Stickle Main Face or Crescent Climb (Pavey), or the pitched section of Pinnacle Ridge on St. Sunday Crag... And, if you climb, you can use it as a gym rope the rest of the time...
But, if you just want to practice knots, any old piece of cord will do! Shoe lace? You can do pretty much everything with that, including prussik knots (if you prussik the lace onto your finger)...