Get some clean sea water
put your worms in a cat litter tray or a biggish bucket so they are not crowded. then get a fish tank pump, clear tube and aerator stones to put loads of little air bubbles into the water.
Keep the water well aerated and cold and they will last you 3 weeks, no probs.
check them daily, change the water every 2 - 3 days if its still crystal clear, if it starts going discoloured empty it before then, find the bad worms and put them in paper (glossy mags are best)
water in the trays or buckets only needs to be 50mmm deep.
if you cannot get a pump set up, just change the water 1-2 days.
hope it helps
Paul