As others have noted,
Home — TreasuryDirect is the way you can directly buy ibonds. It has been pretty seamless for me, but I do understand that some people have reported issues. It has become very popular, which may be stressing the site.
Ibonds are limited to $10,000 a year. You and your spouse can both have an account, which allows $20,000 per year. As others have also noted, you can buy treasury bills for larger amounts and variable time frames, with fixed interest rates for the selected time frame.
A good source of information on both of these vehicles for me has been the Wall Street Journal. They have published a number of articles/videos, and you can search them with an online subscription. A subscription is pricy after their trial period, but if you have a library card, you have access to the on-line version for free, at least for the Sumter county library system.