Go to where the problem is already being complained about, and read before you speak.
The places that work:
Wherever people ask for help with the current bad solution. Forums, communities, support channels for the tools they use now. Somebody posting about a workaround is somebody with the problem, right now, describing it in their own words.
Reviews of competing products, especially the two and three star ones. Five stars tell you nothing; the middle ones are people who tried and were disappointed and explain exactly why.
Job listings, if your customer is a company. Roles being hired for tell you what somebody is trying to solve badly enough to pay a salary for.
Local or professional groups for the trade in question, if your customer is not a software person.
The important part is that you read for a while first. You will learn the vocabulary, and the single biggest reason cold outreach fails is using your words rather than theirs.
Then approach individuals, not the room. Posting can I interview people about X in a community gets nothing and often gets you moderated. Messaging one person about the specific thing they wrote gets a reply surprisingly often.