Real estate investing question dealing with partners/investors?

I own some investment property and have had a lot of luck with it so far and want to continue to invest but do not have enough to purchase something else by myself. I was actually looking at small apartments and quads. I was wondering how to find an investor(s) to fund the initial down payment, I manage the property (plan to do a llc so I have an interest and can legally be the property manager) and I eventually pay the investor off with interest after 3-5 years. I know about angel investors but their terms and interest rates are ridiculous. Any websites with info would be great.