Vivian O'Neill, author of The Healthy Pet Herb Guide, has used the following herbal supplements to strengthen kidney function and break up calcium deposits:

Kidney Activator
Alfalfa
Joint Support
Devil's Claw

If you suspect there may be a urinary infection, she highly recommends JP-X

According to The Healthy Pet Herb Guide, dosages will vary according to the weight of the animal. A general rule of thumb is 1 capsule per day for every 25 pounds of body weight for both dogs and cats. If in doubt, start with less and slowly increase the dosage.

I hope this helps!

P.S. Vivian has a new "companion" booklet out now called "More of the Healthy Pet Herb Guide", which contains many case studies from her animal clinic. It's an excellent addition to her first book, and I highly recommend it.