I made it myself. I bought some ribbon at Walmart. key rings and bought big bells at Michaels. Had maybe $4.50 in materials. Check this site out
www.poochie-pets.net/
They definately work. The bells that I bought are so big that you can't help from not hearing them. Everytime Sydney rings them we all come running to let her out. You can tell by how she rings them by what she needs to do.
Hope they work for you.