You can always buy an used Pixel. GOS only supports phones with hardware that they deem secure enough, and Google is currently the only manufacturer that makes phones that match their requirements[1], with upcoming Motorola phones that also support or even come with GOS installed coming out next year. I find this point so obvious that I can't understand people making comments like "can't take them seriously": you make security/privacy OS, having insecure hardware that compromises the whole project would simply not make any sense. Ask other manufacturers to do better.
1: https://grapheneos.org/faq#future-devices