Where do i get dried flowers from for invites?

I've used a place on Etsy called Lytton Rose Botanical for dried flowers (venue decor and a bunch to send to my sister for a birthday!) - they were gorgeous and lasted really well.

Sounds like a lovely idea you're doing

Etsy is a good shout, or if you have time you could press some yourself and use a petal from each on the seal, or you would need small ish ones to get them to stay in the seal without damaging in the post,