Mitosis and meiosis have very different purposes. If you understand the purpose of each it will help to determine which plant tissues to look at.
Mitosis is the process of cell division where one cell is copied, creating two identical cells. It is always used for growth and repair. Consequently the best place to look for cells undergoing mitosis is the meristem, or growth areas of the plant. These are found at the tips of roots and shoots, inside buds, and in the cambium tissues.
Meiosis is only used for one purpose, and that purpose is reproduction. In meiosis a diploid cell having two sets of chromosomes is divided into four haploid daughter cells whcih each have only one set of chromosomes. These haploid daughter cells then differentiate into gametes, the reporductive cells. In order to find cells undergoing meiosis you have to look at the areas where gametes are being produced. Egg cells are produced in the ovary and sperm cells, which are found inside pollen grains, are produced in the anther.
The only plant tissue where you will find both cell division processes occuring at once is inside a flower bud.