Astilbe is an amazingly varied genus. I was looking for what you're describing several years ago and was given a plant called "sprite". Imagine my surprise when it was about 8 inches tall!!

I just found Dayton Nurseries on the internet. They sell
plants and are in Northern Ohio but I'm just suggesting
that you look at what's there to get an idea of what species or variety you're interested in. I can't lay my hands on my Thompson & Morgan seed catalog but when you have a name you can see what is available as seeds.

I lust after Astilbe thunbergii "Ostrich Plume" but I'll
have to fix a rich enough and damp enough spot before I
try it. I have a very nice fluffy Astilbe arendsii 'Bridal veil' which survives in not so damp places.

Good luck, Chris