The endgame fleets, war fleets, etc. are not trivial to set up because they all require custom events to be done properly, or it just feels unprofessional. I simply don't have the time to make them. Until then, high-end bounty fleets are quite difficult indeed.
The Templars will have an event that spawns non-Templar war fleets from nearby systems, however. If you activate that mod once I finish the update, you might be able to fight some big fleets.
Cathedral, Infernus, Monolith, etc. can show up in max-size patrols on occasion. Just attack them long enough and you'll see them pop up.
Renegades will show up at some independent markets, at black markets, at pirate stations, etc.
The next update later today will probably boost the max size of bounty fleets, fix a flagship bug, and add small refineries across the sector that add fuel into the system. Keep in mind that fuel use is only doubled currently, not quadrupled. And if you change that setting yourself, make sure you also modify the comm relay stats inversely proportionally.