What you want will not work. Sorry to be so blunt but think of carrying in a dandelion puff. The first wind from a passing person will destroy what you are trying to give her. May I suggest that you find a dandelion puff in acrylic and present as part of the cocktail hour telling that with the gift you wish her the joy of the dandelion puff. That way when things go rough she can take it out and think of the joy they shared. that day.

Yes I do get wishy-washy at times.

The Common Dandelion is Taraxacum officinale and seedheads ripen in May.
You won't get much sympathy on this forum for this weed. It was introduced from Europe probably in hay or bedding for livestock.
Why not consider a native plant, would Common Milkweed work?
Sam

Dandelions often rebloom in the fall, so there will be seed heads sometime before the wedding, but I am not sure if they will be ready in October. I suspect there will be, but I have never paid close attention to exactly when they bloom here in PA in the fall. If they are in season there will be enough of them that almost anywhere you are you could find wild ones and pick them. However, I agree with maifleur that they would be very hard to handle. Perhaps you could manage to get a small number into the reception area intact, but I doubt they would last long.

Good Luck!

I agree that finding dandelions gone to seed for a specific date in the fall and transporting them intact would be a nearly impossible task.

How about other "wish" themed objects: glittery stars on a wand? Cleaned and boiled chicken wishbones spray painted in the wedding colours? "magic rings" (dollar store trinkets), coins and a table fountain? Genie lamps or bottles?

Here is a link that might be useful: Make a Wish!