There are plenty of 15 year Treasuries: 30 year Treasuries with 15 years left on them.
You can also find what rates are for for them by checking the yield curve.
So the answer to your question is that Treasuries maturing in Feb 2031 yield 2.495%.
In practice you can repay your mortgage if rates fall, but the feds cannot. This option value requires you to pay a higher rate, so the numbers are not comparable. Prepayments also mean mortgage rates are more closely correlated to shorter term Treasuries.
