Usually, that’s emailed to you as part of the transaction. Might have been sent to your spam box, perhaps, worth double checking. Assuming they have the correct email address, of course. 

Sonos sales doesn’t maintain a presence on this community website, so you may be better off calling them directly and asking for one. You could ensure that they have the correct address, too. 

Search your email for Sonos Commercial Invoice otherwise you could contact the Sales Team to ask for a copy to be sent to you.

You can log into your Sonos account on the main website (click the user icon), then go to the orders tab.  View for the specific order will give you receipt/invoice details.