I would like to buy some of the plants for my islands. Is that possible?

Not sure what plants you are referring to. You can buy plants in the regular store, and sometimes there are some in the seasonal (monthly) set. If you are referring to the bushes that you place on other people's islands, then no, you cannot purchase those for your own island. You can only receive them as gifts from others. Some players coordinate with their teammates or Facebook friends for specific placement, since they cannot be moved after they are placed.