How to extract recovery images from existing roms? Or is there a working TWRP site?

The recovery images all over this site seem really sketchy, but roms often come with great recovery and updates. I can flash the rom's boot.img during the update, but I also see that fastboot has a flash to recovery option. Is it OK to flash the boot.img to recovery or do I have to somehow extract recovery out of the boot.img? If I do a flash to recovery does that make TWRP permanent or can it still be overwritten by flashing new roms?

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