How do I find a free cleaner for my infected iPhone?

None needed. That is a scam message. Ignore it. A non-jailbroken phone cannot be hacked, get a virus, or malware.

First, your iPhone is not infected. iPhones aren’t even susceptible to viruses. That is rather a scam attempt message designed to install unnecessary software with your money or give up your personal information. Two, as a result you do not need a cleaner app. Those do not perform as advertised and are rather useless.

You don't have any viruses on your phone, it is a scam message designed to get you to purchase unnecessary software or phish for personal information.

Read more about it here -

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

Even if you could find a "free" cleaner.......it would still not be worth the money.