Pancetta *is* bacon from the belly (the word comes from pancia, belly).
Like guanciale (from guancia, cheek/jowl), it can be relatively fat or
relatively lean, it depends on the cut. Texture-wise, pancetta is
perhaps the best substitute. If fat is important to you, I'd suggest
cured fatback which, however, can also range from almost all fat to
somewhat lean.
By the way, since you cannot find anything under the name of guanciale
locally, you might also want to look for goletta, barbozza, barbaglia,
or barbaja, all of which are the same thing.. even in America Italian
grocers could be using regional names. I once posted on the subject of
guanciale:
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https://groups.google.com/forum/message/raw?msg=rec.food.cooking/NRdAs0XGdcU/W9cZCWG4vlYJ>
Victor