It looks like the shortage of Canon EF-EOS R Mount Adapters may soon be over — two dealers are now showing very specific shipment dates for new stock of this adapter.
Amazon is showing that the adapter will be in stock on October 6th. Adorama is indicating that the adapters are expected to arrive by September 25th. A backorder placed at either store should reserve an adapter in this next shipment.
If you need one sooner, this 3rd party adapter via Amazon is $54 and has great reviews. Resellers are asking for $160 via eBay for the original adapter.