Financing A SS..Am I Not Getting It?

I'm a newbie at SS so I am still unclear about financing them... Ideally yes I'll be able to do a double close and close the back end first to finance the front end but if I dont find a buyer who can close fast then I'll need cash. Everybody has told me that if the deal is right the money will come... Well I am negotiating a deal that'll need approx $440k to close (FMV 600k) plan is to sell for 520 - 88% of FMV....but am now finding that many lenders will only lend 70% of purchase price and have a ceiling of 300k plus they want to be the first and ONLY note holder...leaving me with a hefty amount still to find /carry Any suggestions as to how I can approach this?? Some wonderful help on this BB Thanks! JK