The IRS would like an "exact" value but they are happy with a reasonable estimate.
Since it's through Treasury Direct, they won't give you any help. You can find the Wall Street Journal at a library for the day after your father's death. They probably have a price for a treasury bond close to your father's bonds interest rate and maturity date. An exact match may be found. If not, look a few days before and a few days after. The values don't change that much.
The other approach is to use a financial calculator -- HP and TI make them. You can enter the approximate prevailing interest rate, the cash stream from the actual bonds, the maturity date and a present value can be quickly calculated.
It's all close enough for government work.