Radio Shack comes to mind, if you take the adapter to them, they have collection of reference plugs. Just because you go there doesn't mean you have to buy their solution. That one of the few uses that radio Shack still handles well is the adapter plugs.
Myself I just try similer ones until they match making sure the polarity is correct first with a volt meter but positive is typically the inside contact.
Another option is to call Tivo, they might just send you a replacement at no cost. I have seen others post that after moving they lost the remote control dongle and after calling Tivo, they recieved a free replacement.
TiVo Roamio Pro
TiVo Roamio Plus (3tb)
TiVo Mini (three)