If you're using another platform to sell or acquire users, you may be able to pull those emails out and manually import into your Email Service Provider list. Examples: Etsy, LinkedIn, Gmail contacts, Shopify, etc. This may take some time, but it will be worth it to build an instant list of qualified subscribers. Be careful to properly explain where the email came from when you first send out an email blast to these people, or you could see large unsubscribe rates.