Along with ordering the new Canon EOS R5 and EOS R6, many photographers ordered compatibilty for their existing EF, EF-S, and TS-E lens kits in the form of one of the Canon Mount Adapter EF-EOS R variants. The order volume for the basic and control ring versions of this adapter were so high that they are out of stock nearly everywhere, with B&H indicating 4-6 weeks for availability (7-14 days for the control ring version).
While these mount adapters can be found, such as on eBay, most are selling at a premium price. Alternatives include getting the drop-in adapter (remember that a clear fitler is optically required if no effects filter is installed) or a Vello Auto Lens Adapter for Canon EF/EF-S Lens to Canon RF-Mount Camera.
Canon Mount Adapter EF-EOS R: B&H | Adorama | Amazon
Canon Control Ring Mount Adapter EF-EOS R: B&H | Adorama | Amazon
Canon Drop-In Filter Mount Adapter EF-EOS R: B&H | Adorama | Amazon
Learn more about the Canon Mount Adapter EF-EOS R.