You will find seeds to buy at the stand on Main Street or at the Port. Seeds grow at random into different colours so you'll need to keep planting until you get the colour you want.
To get the black love berries, you must have luck. What you do is plant love berries and see if you get the right color. If you don't, then you should buy another love berry, and keep trying until...
To attract Moshlings you need to buy seeds from the seed stand on Main Street or at the Port. Then, you plant three seeds in your Moshling garden. Each Moshling has a different combination to attract...