Where can I get a replacement drawbar for a bike trailer?
The mounting point is Thule ezHitch axle mount, so maybe it is an old Thule trailer. You can check al local dealer.

But...
I suppose it would be expensive to get that this way.
All you need is metal tube you can get in DIY store.
Undo rivets, drill holes as on the old one and you're done.
Local locksmith service should do it for you for reasonable price.