Reducing threaded rod to mount fixture cross bar
The thread sizes for these mounting fittings are specified in IPS, "Iron Pipe Straight."  It's a hold over from when lights were mounted on repurposed gas lighting, which used the IPS sizing.  Later IPS was replaced with NPS but the nominal sizes are not the same.  Lamps still use the old IPS sizing.  

I believe the size you have there is 1/2" IPS.  Try a 1/2" IPS to 3/8" IPS hickey with your 3/8" IPS crossbar.  I'd not expect to find it in a local store, but it won't be hard to find online.