Try hotwire or priceline
Try a one-way rental
Maybe rent at OAK and return to SFO?
bart is convenient from SFO but to OAK you stop at the coloseum station then you transfer to a bus to OAK Airport, then you transfer to another bus at the terminal to take you to the Rental Car center.
There is a possibility of yet another bus transfer if you rent from Payless or Advantage at OAK.