More places than people assume, but every dollar has to be traceable.
Acceptable: savings, gift funds, retirement accounts, proceeds from selling an asset, and down payment assistance programs.
Gift funds require a gift letter stating the amount, that no repayment is expected, and the donor's relationship. Conventional generally requires family. FHA allows broader categories. Never an interested party, meaning not the seller, agent, or builder.
Seasoning. Funds sitting in your account 60 days or more, meaning two statement cycles, are considered seasoned and generally escape source scrutiny.
Large deposits get questioned, typically anything at or above roughly half your monthly gross income. Underwriters are ruling out borrowed money you did not disclose.
401(k) loans and withdrawals are allowed. Crypto generally must be liquidated to dollars with a documented trail.
Cash saved at home generally cannot be used. It has no traceable origin, and traceable origin is the entire point of asset documentation. If you have been saving in cash, deposit it and let it season before you apply.