Hi Theresa,
In my experience, if the cat moves onto a grain-free meat-based diet such as a home-cooked or raw diet, the protein content becomes sufficient to acidify the urine without the need for additives. A delectable diet will be easier to add herbs to it, in the event you pursue that route. If the problem basically boils down to acidification, Hydrangea and Eupatorium combination usually excels at that. It doesn't taste great, but a small amount mixed in tuna or something will probably work. You can also improve the taste by steaming off the alcohol in the bottom of a warm pan, then keep the herbs in the fridge. Worst comes to worst, the herbs can be given in a gel cap. I'd look at 0.2 ml per 5 lbs BID if you can change the diet, 0.4 or 0.5 ml per 5 lbs BID if you can't.
For advice on cat diets, we just use the guidelines in the appendix on home-cooked diets in the Manual of Natural Veterinary Medicine, the paleolithic one in particular. Vets also like the website catinfo.com for home-made diet ideas. BalanceIT will make a grain-free diet to your specifications if you subscribe to their site, but don't have any for free. Lastly, Donna Raditic sometimes cruises this site and is a veterinary nutritionist (almost, anyway). She may have some further thoughts on diet resources.
Hope this proves helpful. If the cat can be convinced to eat home-cooked or raw, this should be a piece of cake for you to fix, no pun intended.
Steve