Any short, distinct sound should work the same as the clicker.
Dog trainers usually use clickers, whale / dolphin trainers use a whistle and goldfish trainers often use a flashing pen light or a training wand.
(yes goldfish can be trained! see a video here: http://stalecheerios.com/blog/2009/02/fish-training-progress/)

Studies have shown that a real clicker works better than using your voice. However, if you can't find a clicker, pick a sharp, distinct sounding word and say it in a tone different from how you usually talk to the dog. ("yes!" is commonly used).
Here's a good article on why using a clicker can be better than using your voice:
http://www.clickertraining.com/node/275

You can also make a clicking sound by pressing your tongue against the roof of your mouth — this is what I usually do with my horses.
References :
I train horses, dogs and goldfish, all using clicker training and positive reinforcement

Just checked with Ebay UK and there are loads – presumably somebody would ship… might take time however.

Hope this helps.
References :