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.
And consider GrapheneOS.
Anyway, please read GrapheneOS.org in its entirety.
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.