I used one for my boy when he was in an outside kennel. It really worked quite quickly for him. I don't use it now as we have a puppy and I thought that it would be unfair to my adult dog who knows not to bark if the puppy barked and they both got told off.
I think that mine was about £60 and it was from this website
http://www.zencatalog.com/PestControl/pest_control_store_Ultrasonic_products_for_the_home_37.html (sorry dont know how to do links) and it was the Super Bark Free Bark Controller PET013. It is good outside and seemed to work well.
I don't think that it is ideal but I was desperate and used it alongside other training.
Hope this helps
GG