Actually, I think if you buy flowers from that NPC she will eventually sell those. I was buying wijnruits for alchemy and she 'stocked up' on some pumpkins after about 4 or 5 stacks purchased.
I was trying to make some fireworks in my alchemy synths, and this vendor (Dabih Jajalioh in Ru'Lude Gardens) does NOT sell twinkle powder. She sells flowers. Sorry!