Do a market research and search for competition, so you know what are you fighting against. Try to get in trace where are those competitors gain their leads, try applying similar method, but look around for additional sources of audience and potential leads. Most of tools nowadays have narrow targeting, so it won't be too much of a trouble finding potential leads in that particular niche.
Can you run ads? You can run lead gen ads on Google, Facebook, Instagram, Reddit, Tiktok. It depends on what type of leads you need. If it's better for them to call a number you could do it on Facebook, if it's better for them to sign up somewhere and you have a promo video TikTok should be cheap. If you only have an image, Reddit is cheaper and you can target subreddits related to the subject you're looking for. If all you need is a list for cold calling you can use a webscraper, it all depends on the type of lead you're trying to convert.
