Where can I find information regarding the number of units of a
certain product that were sold?  For example, I want to know how many
pairs of shoes or how many cell phones were bought last year.  Where
can I find this type of information?  I have already tried the Bureau
of Economic Analysis(www.bea.gov) and this site gives me total
consumer expenditures for shoes but does not tell me the number of
units actually bought.

These issues are hard to come by, and I don't think that there is one
source for all kinds of products. You might want to try Google Answers
with specific products, though.