You can try tools like OptinMonster, LeadPages, or SumoMe for email optins
However, it's important to find out why your visitors are not converting. When visitors don't convert it basically means
1. They don't understand the content on your landing page
2. They understand your content but can't find what they're looking for on your web page (for example, a specific feature about your product or service that is not mentioned on your landing page)
3. You're attracting visitors who are not your target audience, so your product or service is not relevant to them
I suggest adding a chat widget to your landing page to find out who are your visitors, what they're looking for and what they're unable to find on your landing page.
Once you create a landing page with relevant and useful content, people will naturally convert.