They are pretty sturdy plants, so there may be hope. If it's too late this year - hang on to the seeds for next year.
Maybe WalMart garden shop may have one in bloom. Call your local agriculture extension center or a 4H club member to see if they might know where you could find one.
If the seeds take - she will have a whole bed of beautiful sunflowers.
Hope you discussed this with the little hoodlums' parents.
I just spoke with a very nice lady at Linder’s. Unfortunately it’s too late in the season for the seeds to germinate and take, plus the seeds may not be “ripe”. She told me what to look for in the seeds to know if they will be viable at all, but said they don’t transplant well so that may not be a good option either.
Short of finding one and transplanting it though, I fear I have no other options - it can’t be grafted since it was totally broken off.