While renovating a condo bathroom, we removed the old rectangular sheet metal ductwork and replaced it with 4" insulated flexible to gain valuable ceiling space. The old diffuser was a 2-way 6x8. Is there such a thing as a 4" round to 6x8 adapter that would allow us to reuse this diffuser, or I am looking at replacing this diffuser with say a 4" round.

If you can't find on in the big box stores go to a sheet metal shop and have them make you one. Simple thing for a sheet metal mechanic. Called a transition.

Can't find one online or in a big box; they all start at 6" rounds. Any idea what a shop would charge?