Where can I buy GOG games?

Excuse me, I'm wondering if you could point me in the right direction. I'm interested in purchasing some games from GOG, but I'm not sure where to start. Could you please tell me where I can find their games for sale? Are there any specific websites or online platforms that specialize in selling GOG titles? Additionally, are there any precautions I should take when making a purchase to ensure that I'm getting a legitimate copy of the game? Thank you for your help.