SQLPLUS query to CSV output

I have a database where a query on one table results on many records being returned. How do I get the output into CSV format - e.g. "select foo,bar from sometable where foo = '123456789';" should return something like:
   123456789,111,222,333,444,555,666,777,888

The values 111, 222, 333, and so on are all vaues returned by running the query in the above sample SQL statement. Normally, SQLPLUS returns something like:
   
     FOO                BAR
   ----------------   -------
     123456789      111
     123456789      222
     123456789      333
...and so on...