Boy, you're not alone. Finding low-cost dental care is tough anywhere.I'm going to suggest you do a quick internet search with the terms "low cost" and "dental care" and Phoenix. I just did this and came up with several sources. Here are a couple:
The A. T. Still University Dental Clinic
John C. Lincoln Hospitals
I don't know details about either of these places, but they look legitimate. If they can't provide care, they very well may be able to refer you to who can.
I'd also try the U.S. Government's Eldercare Locator; an on-line (or phone) service that connects you with the Local Area Agency on Aging in the region you want. The Local Area on Aging can make referrals to senior services in Phoenix, including for dental care. Best of luck.