The best site for that is GuildStats, as a bosshunter I'm using only this because it's most precise.
For example:
You can see here predictions for Zarabustor, it says how many days it's on high chance and which day he was most often killed (so % chance for his spawn also).
Akso there are bosses with more than 1 spot so guildstats is not the best option for that but you can try TibiaBosses but it's still only high chance