--  

You can't really just buy
finished cushions unless you find some selling some that are just the right dimensions.
You should get the webbing redone and then see what density of foam feels best on back and seat. Then have the upholsterer make cushions. They should be 3" thick for the seat and 2" for the back, or 4" and 3" if you like a chunkier look.
Boxed seat cushions with welt are something that most upholsterers do all the time. They're the same construction for Danish Modern chairs as they are for a traditional chair or sofa. You just have to find the right fabric. Maharam/Kvadrat still makes the fabrics that were on the original high end Danish furniture in the 50s and 60s. Not that Baumritter would have used those fabrics. But it's not that crucial to the value of these chairs. Just find something you like that isn't too synthetic. And no chenille! Chenille has been all the rage for at least 10 years now but it wasn't used when your chairs were made.