1.) Is it safe on Amazon?
2.) Which other shops can you recommend?
(Wanted to create a Netflix account for me, but PayPal was not possible and I did not have a credit card)
So I have no idea about all the shops, but if you don't know what to do, just go to a petrol station near you, they usually always have something like that.
Yes, it's safe enough on Amazon. However, you can also go to a supermarket and buy the cards there. Would currently prefer Amazon, however. I would not make such purchases on eBay because you usually only pay before the goods are handed over to you.