If you absolutely can't support SSL connections on your origin server, you can use Flexible SSL. If you choose Flexible SSL from the Cloudflare SSL options, you are not required to have an SSL certificate installed on your server.
Recommended for maximum security, we strongly encourage choosing Full SSL or Full SSL (Strict) settings, both of which ensure that requests between Cloudflare and your server are encrypted. Full and Full (Strict) require an SSL certificate installed on your server.
This article explains the different settings in more detail: