Need to translate your Post into English since AMD Forum is a English Only Forum.
Found this old Thread here on AMD Forum concerning the same laptop and HD 6970M card replacement.Modern alternative to Radeon HD 6970M
I have checked several website including Ebay none sells Samsung HD6970m replacement cards.
best bet is to contact Samsung Support and ask them if they still sell HD 6970m replacement video cards for your laptop. Your language seems to be German, So here is Samsung.com in Germany which you can call or chat with about your problem.Hilfe und Support für Produkte | Samsung Service DE
If the IMAC HD 6970m is different in shape it may not fit in the laptop correctly. Also the HD 6970m may have been customized to work with a IMAC Laptop.