GeoTrust and RapidSSL both offer coverage for www and non-www. As long as the certificate is generated with www as the common name, the non-www version will automatically be covered.
This is not the case, however, for Symantec and Thawte certificates. You will to purchase separate certificates to cover both the www and non-www common name for either of those brands. Comodo certificates also automatically cover www and non-www.