Where could I buy an iPad Pro with the M2 chip?

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You could check the Certified Refurbished section of the online Apple Store, to see if there are any there.

Selection there is "catch of the day" – so it may, or may not, include the models and configurations you are looking for.

Apple directly markets only "current" models through its physical and online stores. Earlier models, such as the sixth generation iPad Pro (with the M2 CPU) will be available through Apple Authorised and independent retailers while stocks last.

Otherwise, as others have suggested, you might find a reconditioned/refurbished iPad via Apple's Certified Refurbished online store. Some Apple Authorised Retailers are also known to carry Apple's Certified Refurbished stock.