I agree with Cary., AMSI will be your best source for short term leasing in San Francisco.
As well, you might want to check with other rental management companies, such as William Hill Property Management, http://williamhillpropertymanagement.com/
San Francisco Realtor
Good one! You don't need a Real Estate Agent, you need a magician . Just kidding, kinda of. Major factors to consider are term and location, both are at a premium in SF. With that said, the majority of Real Estate agents are sales orientated, not rentals. It's apples and oranges. So to find a firm that can do both, can be taxing. At AMSI, we cover it all. We have our Roommate Connection program which is at no cost to you, until we match you up with someone with similar needs, and have accommodations available. By combining your renting power with a roommate, you could probably get you a furnished unit, but there again, the term of 6 mos will be a factor. There is Craigs List and SRO units (Single occupancy units) available for the City, but please, do your due diligence. As always, myself and AMSI are here to assist in any way possible in finding accomidations for you, just call. Below is my contact information. Feel free to contact me or my office with any questions or concerns.
2800 Van Ness Ave.
San Francisco, Ca. 94109
America's Cup Housing Provider
Web : http://www.amsires.com
DRE # 01319108