I think I've seen physical Amazon gift card for the UK store. In fact I'm positive I have because I've sold them in the past
To be honest, I don't think anyone would sell legally obtained gift cards for 65-80% off seeing as people pay much more for them elsewhere online.
Usually the cheaper ones are sellers looking to dump their codes. I think you'd be looking at having to pay up to 90% of the face value. I could be wrong though.