any pet store will sell these in store, many also on line. try www.petsmart.com for bark control devices. Look for the pet safe sonic bark control products.
How they work... the device will "hear" the dog bark and send out a sonic pitch. This does not hurt the dog, but will get the dogs attention. At this point, you should use a firm "no bark!" command.
BEFORE you spend the money, you should get a cheap dog whistle. When your dog barks, use it. If your dog responds to the whistle, then the sonic products will work. If your dog doesn't care, don't waste your money.
Remember, these aren't cures, they are training tools. Whenever you use any behavior modification device, you MUST accompany it with training. If not, the dog will learn what triggers the device and stop, or will develop a tolerance and eventually it won't work. Hope that helps! Those things are too much money of they won't work for you.