You probably will have to return the whole unit. I doubt the seller would be setup to replace just the PSU. Why don't you call them and ask?
Newegg requires you to pay the return postage, Amazon doesn't. I returned an OBi110 to Newegg and it cost me about $10. For $10 you can probably but a replacement.
I have a couple of 12V PSUs from an old modem and router. They seem to work with the OBi110. I reverted back to the original PSU, so I did not test long term.