Mario Batali's dad, Armandino, started Salumi Cured Meats some years ago. I understand he's retired from the day-to-day running of the business, but the company lives on and is doing well:Guanciale
La QuerciaHome - La Quercia, LLC
Niman RanchNiman Ranch: Guanciale
Chris Cosentino's company, BoccaloneCured Meats
Salumeria BielleseCured Meats
There is supposedly another company local to my area, but I've not confirmed that they sell guanciale, so I won't mention them.Edit:
OK here y'go:
The Fatted Calf - San Francisco & NapaFatted Calf Charcuterie
Also, Esposito's pork store on 9th Ave in NYC
The product is also available through several sources in the Los Angeles area, but I've not dealt with them. I'm sure that you can get it from any place that sell's Niman-Ranch products and Salumi products. La Quercia provides product to several on-line retailers, one being Zingerman's.
These are just the places I know of or that I have purchased from - I'm sure there are plenty more.