We used one from Träumeland and were able to sell it on when we no longer needed it.
The bed, on the other hand, had to be disposed of afterwards. The mattress was the best thing about this bed.

The mattress was the best thing about this bed, and it didn't cost a penny more, so I think it's a good idea to invest in a good mattress, given that this bed only has a rigid slatted frame.

I have one from Träumeland and one from Landi. I think that expensive is not automatically better.

We have an Ikea mattress 70x140 and it fits wonderfully. According to KTipp, Ikea has relatively good and inexpensive children's mattresses.