When you don't have the coins to buy a habitat it is a struggle to get them. But try to check your island as often as possible to collect coins from your habitats. I don't know if you can mine, but if you have bombs from evolving dragons, the mine gives coins or apples when you find the island parts.
If you visit your neighbors you receive either experience stars or coins, so visit. There are a few players who have islands where you can post SOS messages for others to visit you so you get coins that way. Mommy is one such player....if she is still playing, but you need to stumble upon these players on your visits to be able to visit them
but if you know they exist, it helps.
If you have decor you don't want, like or feel you can get rid of, you can sell those for coins. You need place on island , tap on it then tap move. A pop up appears and most should have option to sell.
Since I don't know your experience level with this, I apologize if you know about most of this already.
GL in getting a pink habitat.