Italian deli and yes it's extremely expensive. Plus, I can feel my cholesterol rising just looking at the package. I can find pancetta in most supermarkets in my area but I have to go to a specialty store to get guanciale, or, as you wrote, order it online.
Pancetta is a good substitute, it's easy to find and way cheaper than guanciale. If I were to make Pasta Alla Zozzona on a regular basis, well, first of all I'd weigh 400 lbs in no time, but also I'd likely use pancetta. However, this is a dish I plan to make once, maybe twice a year for a special get together and I can afford to go all out once or twice a year, both in terms of the financial and the state of my arteries.