The new iPad Pros and iMac will be pre-orderable from tomorrow, but Apple has hinted that it will not have many units available. Apple expects shortages to continue into Q3 2021, although primarily for iPad and Macs.
https://www.notebookcheck.net/Apple-expects-iPad-and-Mac-shortages-ahead-of-new-iPad-Pro-and-iMac-launches.536118.0.html