You can also buy them in the secondary market with known yields. Generally, short term T-bills have a pretty predictable auction rate.For anyone that has never bought t-bills before, they are actioned off, so you don't know exactly what the interest rate will be until the auction takes place and you have already placed your order. Big investors bid a price they are willing to pay for the t-bills and that sets the price for everyone else.
I believe there is a treasury website that posts the most current rates, but I think it is the rate from the last auction. What you will actually get is usually fairly close to the previous auction rate.
It is also much easier to place the order through a broker than through the Treasury website.
If anyone needs help, I can point you to a youtube video that explains everything and walks you through placing an order.
If you open an account at Home — TreasuryDirect links to your bank account, you can buy savings bonds, t-bills, t-notes, t-bonds, tips, etc. free of commission. Also, they will deposit the interest directly into your bank account and the principal when you security matures.