Hi,
Try to find a small highstreet ironmongers, failing that even a B & Q's maybe?
Sometimes the smaller shops will sell you the sort of lengh you need and the joining link for mere pennys so have a look around your local shops first.
Just briefly on your presentation worries, you could present it as a warning demonstration, or even showing how magical abilities can be used to outwit the con man, alot of course depends on your own chosen style. David Roth (yes the coin man

) had a nice routine that put two spectators in compation, man/wife boyfriend/girlfriend etc.
Have fun,
John.