Guys - Most of the so called "paracord" I've ever found in the shops is rubbish. I suggest going to any shop that sells climbing kit and asking for accessory cord.
This is a bit nerdy but in case anyone's interested :
"Accessory cord" is normally nylon (polyamide) because it will stretch and absorb shock.
Anything 3mm and above is made "kernmantel" style - i.e the white stranded core that provides the strength, wrapped in a woven sheath that provides the wear resistance and handling.
Minimum breaking strengths are typically :
3mm = 180Kg = what I use instead of paracord.
4mm = 330Kg
5mm = 580Kg
6mm = 750Kg
7mm = 1050Kg
8mm = 1400Kg
9mm = good enough to save your neck falling off a rock face
2mm is good for putting lanyards on light stuff like whistles and compasses. It's good for 70Kg though, so it's strong enough to strangle you and too strong to snap bare handed.
Generally you buy by the metre off the roll. Watch out for the price - 40m x 2mm or 20m -30m x 3mm can sell for under a fiver, but some people mark it up pretty heavily.
Exotic spectra or dyneema cord is v dear, but gives an amazing 1800Kg tensile strength in a 5.5mm dia cord which doesn't stretch.
Cheers