How to find Syndicate hideouts?

I have just completed main story with my Trickster but only encountered Syndicate hideout once. I need to get Catarina as my character requires The Devouring Diadem. How can I achieve that? Where are the Syndicate hideouts?

You'll need to complete Jun missions in maps. The wiki explains how it all works.

https://www.poewiki.net/wiki/Immortal_Syndicate

Like most mechanics, the Syndicate is mostly something you handle post Campaign. If you're hunting for a good Diadem, expect to be at it a bit.

Step one is do maps of any kind, preferably ones you've never gotten the skill point for. You will end up finding some Syndicate events during this. Utilize the bonus Jun missions you gain along the way to speed things along.

Use the Atlas Passives you gain to secure some of the Syndicate Altas Passives to speed it up. I'm a big fan of Test of Loyalty and Intelligence Gathering for aiming for big Catarina fights, though the latter its pretty deep.

Thanks. So basically it is a luck of the draw if I get the Jun mission until I can get higher chance of it via Atlas? The biggest problem for me will be getting higher tier maps done without the headgear I need as can't run Grace at the moment. May need to find some mana reservation gear/passive skills.

One more question about Catarina fight. When I defeat her and she doesn't drop the loot I need, will I be able to do the boss battle again straigh away or will I have to clear a number of hideouts first?

You have to re-do Interventions until you get back to the boss.
It is not a rinse and repeat fight.