The savings from buying tanks online is offset by the cost of shipping. Craigslist is usually loaded with them, but buyer beware. People want way more for their used tanks than they are worth. You never know if there are leak, or if any included equipment is properly functioning.
Petco typically has an annual $1 per gallon sale on tanks up to 55 gallons. It is tank only, not the starter kits, but its another option.