My mix for the past few years of flips or rental rehabs:
Unsecured Lines of Credit (have $50k available).
Various Credit Cards (especially those with no interest for 18 months or big percentage back).
Had a baby just before starting current rehab, called the hospital and asked if I could pay the bill on a payment plan, $10k "loan" interest free paying off at $350/month. I'm part of a healthcare sharing program instead of normal insurance, so I get reimbursed the $10k within a couple months.
Some bills, including some property taxes, assign a fee if not paid on time, that amounts to a few percent for paying later. I think it's a bit of a scam by them, but it is basically a loan then from them for sometimes cheap.
Sold vehicles I had paid off and got a vehicle with 100% financing (could also refinance vehicles you own I think, but needed a bigger van anyway, so worked out).
More credit cards and lines of credit from parent are available.
Refinance on personal home.
I think the most important part of financing is getting a great price on the home(s), and a great price on the labor and materials to rehab it. You don't need that much to buy and rehab a home really. If you don't have that much, start with something small and work your way up.