Private Lending to cover your down payment

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A little bit about me -

I have one 3 unit multi family in upstate ny. Albany Ny to be specific, I purchased this back in April 2019 and provides me on average 20% cash on cash, little over $300 a door.

It would provide me more if I wasn't paying off my HELOC which I used as for purchasing the house (down payment and closing cost) so theoretically I purchased the property with $0 out of my pocket.

Now I'm hungry for more but hitting road blocks in finding financing, traditional lenders (primary lenders) require minimum of 20/25 percent down for investment properties, is it possible to get a secondary loan to pay for that down payment / closing costs? If so, how? And where should I be looking for them and what should I be asking lenders in regards to it? I don't want to come off as cheap by asking. I'd love to hear from people and their tips and processes they've put in place. I am looking for partners but also iffy on how to structure deals if it's equity partners.