Most places like Dixons and PC World will sell short patch cables, but these are probably not going to be much good to you.
I found Maplin sold 12 meter lengths bagged up and got one there (make sure you get the straight-through, not the crossover). If they don't sell any that are long enough for you you can buy a run as long as you want off a drum but you will have to fit your own plugs to it. To do this you'll need to buy the tools, which works out expensive for a one-off.