I have found out that you buy the amulet from Maramal in the temple of Mara. I have bought it a few days ago and it had disapeared the next time I played. Now he wont sell it to me anymore. I thought you had to be there at a certain time. Is there any other possible ways of getting it?
Yes, the Amulet of Mara is regular loot, and can thus be found on humanoid enemies, and bought from shops / found as treasure.
If you're on PC and desperate for one (I assume you're trying to get married?) you can also generate one via console commands, with item ID
If you do the Book of Love quest(given by Dinya Balu of the Temple of Mara in Riften) you get a free Amulet of Mara near the end(you use it as during the quest, but after the quest is over it's no longer a quest item and you can do whatever with it). There is another guaranteed Amulet of Mara in a tent along the northern coast of Skyrim. Go to Dawnstar(carriage is 50 gold if you haven't been there yet and don't want to hoof it). From the ship by Quicksilver Mine, follow the coast east past the town and Dawnstar Sanctuary. A bit farther east along the coast is the tent with bedrolls inside and an Amulet of Mara between them.