aside from various used models (OnePlus 6, Pixel 3a, various old Xiaomi devices, etc.), the Fairphones are your best bet when it comes to Linux phones (as you specify non-Android based projects). The Jolla phone might also be something worth looking at!

iodeOS, e/OS, GrapheneOS, and CalyxOS (they've come back, hooray!) are all great options if you want a degoogled Android ROM (but is still based on AOSP, so not fully independent) and are well supported on the Fairphones and Pixels, as well as a few other devices. These degoogled ROMs are the best option we have now, mobile Linux needs more time in the oven, and will give you more control over your device compared to stock Android! (don't let perfect be the enemy of good and all that)