I recently purchased some hex power bits from grainger (www.grainger.com) to replace the ghetto rigs I had set up for my hex bits (1/4" to 3/8" shack adapter to hex socket heads). Anyone using this setup knows it will get annoying because the socket head will frequently come off the adapter (even with tape attaching the two). It's kinda hard to find, but there does exist straight up 1/4" drive to 5/16" hex and 1/4" drive to 7/32"
Here're the ones I got from grainger (they're cheap too...about 2 buckets per bit)

Just search 5/16" or 7/32" hex power bit