You can just buy a used phone that's supported by them and install the OS yourself. I have done it with LineageOS and it's not hard.

If you're going to do this, buy a Pixel so you can have the bare minimum in secure hardware.

And consider GrapheneOS.

I'm sorry I don't get why you're downvoted. In what way is a Pixel phone more secure, hardware-wise?

Pixels are pretty much the only Android phone that supports full verified boot with custom keys, meaning verified boot of the entire base system works no matter what OS you're booting (as long as your OS supports verified boot in the first place). They're about on par with iPhones security-wise, but Pixels support alternate OSes which is really nice.

Other Android phones often either have broken verified boot implementations (such as only verifying the kernel) or completely lack it.

The Titan M makes Pixels even better; it is a security chip which stores keys such as those for verified boot.

The security of Pixels is why security-focused mobile operating systems like GrapheneOS often target Pixels instead of say, Samsung Galaxies.

Just to correct myself, about Titan M: It does more than just store keys, it also does other things that contribute to security.

Your guess is as good as mine. Misdirected hatred at Google services privacy bleeding into this debate?

Anyway, please read GrapheneOS.org in its entirety.

You can follow this guide if you have one of the easy installer compatible devices: https://doc.e.foundation/easy-installer

Or on any other supported phone (see this list: https://doc.e.foundation/devices/) you can follow the install guide there. Might even work with your current phone!

The "easy installer" has very limited hardware support at this time.

The first time I followed the install guide, I ended up with a "hard brick" --- wouldn't even power up. Contrary to popular opinion, it is possible to recover from this but it does take time, effort and determination.

As a result of my experience, I made suggested changes which were incorporated into the guide for my device.