You may be able to find citrus plants at your local garden center. If you don't see what you are looking for they may be able to order one in for you. You can also purchase them online from several online retailers.
I bought my Meyer Lemon at the Home Depot. Choose trees that are at least 2-3 years old if you want a tree that will product fruit right away.When choosing your plant look for a healthy looking tree with and sturdy trunk no signs of disease.
If you get less than 6 hours of sunlight per day your may need to supplement with a grow light. If your tree does not get at least 6 hours of sunlight per day it will not produce fruit.
You can fertilize your citrus trees about once a month. Citrus trees require a steady supply of nitrogen so choose a fertilizer with nitrogen as well as iron, zinc and manganese. Compost tea made from worm castings is also great for indoor plants.