How do you guys know so much about deal flow?

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Usually it's just recycled (and low quality) info but you can always get some insights from sources like Mergermarket, Dealogic etc. based on industry/vertical

word of mouth and drawing conclusions from announced deals and media articles on the topic.

I would say Bloomberg is the best source outside pitchbook or mergermarket. You still have to piece some info together but lots of people just echo what they hear other people say.

To your point, you have to pay for most of these services. I don’t know if big 4 outlook and recaps on M&A specifically will talk about specific groups but a helpful free resource.

If you're in a coverage group, then I would think you know the lay of the land of who dominates your industry vertical. For product groups, public news sources said already such as Bloomberg 

Prnewswire.com

Seriously, no one has any clue what they are talking about. I love prospects asking about deal flow because they have no idea what it means. A few big points here:

1) even in a coverage group, there are so many sub verticals, you will have no clue who has good deal flow. Much of it is based on MD’s and they constantly move around. 
2) No banking analyst does enough pitches in enough deals to actually know the space. Be incredibly skeptical of even a full time banking analyst that acts like they understand who has good deal flow. 

3) On the PE side, it’s easier to tell—you can actually review or literally count how many cims you have been sent, or just have a general feel for who does mandates for your firm for the space and who you consider when choosing to pick an advisor. 

Dealogic has a feature where you can cut the league tables a ton of different ways by bank, region, industry etc and even see what deals specifically they had done. That's mainly how I did all my research when lateralling.