Home equity loans or cash out refinances. What you pick is determined by your financial models based on interest rates, origination fees, etc. Typical carrying cost math.

Once you have enough properties to bump up against Fannie or Freddie GSE underwriting guideline limits (~10 properties), you transition into commercial lending, where you build a relationship with a bank and they lend against your combined portfolio.

EDIT: You can find more information regarding these strategies at https://www.biggerpockets.com/

Where can someone read up on these strategies/practices?

appreciated your insight and the link!!