What is the best way to obtain money for a down payment on a rental property? 

I have a great deal that the seller is willing to finance at a 5% interest rate over 30 years with a 5 year balloon.

However, I spoke with a bank that can do a 30-year mortgage with a 15-year balloon at 3.5% interest rate. Far better.

I only want to put down 10% on this deal to keep cash reserves. To get the better deal and go through the bank, how would you recommend getting the other 10% for the down payment? 

I've checked into a HELOC and don't have enough equity built up in our current home.