I did some quick search and found some casinos that are also offering odds on who will be the next pope but I am not sure if you'd consider them as reputable as Stake. Anyway, Duelbits, Sportsbets.io and Betfury offer bets on who will be the next pope(I might add more casinos later).
I am surprised why people do mention more about this other site which has no presence on this forum when there are reputed gambling sites that also have same betting options on this forum. You mention 3 betting sites that also have the bet which makes it 4 sites on this forum. I guess there would be more sites that also has this bet.
Recently I am only using Stake.com and I really like their products and services has been the reason I am lazy to go to other sites, but I know that Duelbit and Sportsbet are also reputed. Also Betfury has been showing presence on this forum for months now and I consider them as reputed already.