If the 1099-B was from a brokerage, call the company that sent the 1099-B and ask. They may have records. It is highly likely they can figure it out, though it may cost some research $ to do it.
As part of that discussion, you may need to figure out how the bonds were held. If they were in the brokerage's name and were held FBO your parents (how most things work at a brokerage), then see the first paragraph. If they were actually held in your parents' name/SSN, then you may have to get in touch with the depository/transfer agent and see if there is a record that can be found by SSN. That can work in some cases.
If you can figure out the day, then you can try to see if sales price records are in something like EMMA. (It will depend on how long ago they were purchased). If not, then you may need to go have a brokerage do research for you (for a fee). Those records exist, it's just that some of them are old enough to require expensive subscriptions or other more strenuous research.
Of course, the alternative is to look through the "everything". I'm guessing I will have an equivalent challenge at some point... but at least I know the bookshelf the "everything" is on...