Loopnet is good, you may also want to try to contact your local Co-Star rep. They will / or should contact you. They generally have field reps that survey the markets for new properties. There also is a few other free submission sites. CIMLS.com is one you can google the rest. Truthfully though Loopnet and Co-Star are probably the two most widely used. Craigslist is always a good bet.