Maybe eBay, or similar sites, just an example:

www.ebay.com/itm/335105008849

But any sturdy parcel box with bubble wrap etc; to protect it and stop it moving about so that it won’t get damaged.

edit: I tend to photograph/video the item and packaging process too, just before sending.

I’ve used the boxes at ‘the UPS Store’ before, for convenience, although I’ve never needed to ship back a Sonos speaker. For a small dude, I think most of them will do the packing for you. 

Thanks Ken.  I went through the troubleshooting with the SONOS rep to determine that the speak was dead but I think it’s a great idea to photograph package before shipment to confirm that it was non-working prior to shipment.  I’m going to get the box from Fedex and ask them to provide bubble wrap as well.  I appreciate the quick response.

Yeah, my concern is that the Move is heavy and will bounce around.  Look at the packaging SONOS uses to protect the speaker.  Anyway I am going to package it at a FEDEX location and photo everything as recommended by Ken Griffiths.  Thanks for the feedback.

Yep, make a box-filling Move and bubble-wrap burrito, couple wraps of packing tape, and ship it off.