Systeme.io
A funnel and online-business platform aimed at building pages, funnels, email campaigns and related workflows. It is more of a software stack than a supplied daily lead list.
GetResponse
An email marketing and automation platform with tools for campaigns, forms and broader audience management. It is relevant when you want to build and manage your own acquisition funnel.
Kit (formerly ConvertKit)
An audience-building and email platform oriented toward creators and owned audiences. The model differs from MLGS because the focus is on building your own subscriber relationship.
Solo ads
A paid-traffic approach where you buy access to another publisher's audience. Lead quality, targeting and economics vary by vendor, so due diligence matters.
Google or Meta ads
Paid acquisition lets you control targeting and creative, but introduces ad costs, testing and campaign-management requirements.
SEO and content
Organic search can create a durable acquisition channel, but it generally requires content, technical SEO, links and time rather than a daily purchased lead stream.
Build your own list
The most ownership-oriented route is to attract prospects to your own landing pages and collect subscribers under your own consent and privacy process.