i want to get the full contact information (i.e., buyer name(s),
addresses, numbers and e-mail addresses) for any retailers in
the united states and canada that sell disposable baby diapers.

Disposable diapers are sold in discount stores, grocery stores, drug
stores, convenience stores, etc etc. There are thousands of these
kinds of retailers in every state.  You are asking for the  contact
information for the buyer at each of these locations?  This sounds
like a very big task to me.  A consultant would charge thousands of
dollars to complete a research project like this, so I doubt a google
researcher would do it.