Cloudflare alternative?

While you're looking into DDoS mitigation/protection services, maybe we should take a look at configuring your server to combat DDoS attempts:

Now of course Nginx has a page on configuring your web server to lessen the damage(s) from a simple TCP flood (someone just sending massive amounts of requests at a time) but this doesn't protect against a botnet sending requests in unison.

For this we have some services including Google's "Project Shield" service which is designed to protect news and activist websites from DDoS attacks being used to censor them:
https://projectshield.withgoogle.com/public/

You could opt for renting a VPS solely to act as a reverse-proxy to your web server, but that won't guarantee the same level of protection as a dedicated service since your ability to set up a reverse proxy for DDoS mitigation is unknown to us at the moment and this is a pretty technical and potentially expensive method of mitigation.