www.treasurydirect.gov. You can buy $10,000 a year per person, although I think it is always a good idea to keep an eye on the Vanguard base money market account as it tends to always be right near the top for short term interest (except when everything is near zero). Also, understand that IBonds are longer term so short term interest rates may be higher at times due to fed action, but IBonds will absolutely protect you against inflation risk no matter what, plus taxes are deferred until you pull it out.