You can look at BIL or SHY etf's, both are very short duration US treasury bonds (in SHY) or treasury bills (in BIL). FWIW I use SHY as the 'cash' portion in my portfolios.
After what happened in '08, I stay away from money market funds.
You don't ever want to hold cash in a brokerage account. When broker's have gone bankrupt in the past, those with cash in their accounts were lucky to get anything back, if they got anything at all. But if all your account is in a security like BIL or SHY, you will be made whole. ETFs, mutual funds, stocks and bonds are in your name and no broker can steal that from you. There's a reason it's called a SECURITY!