Find out the color your gown will need to be. Some high schools require males to wear a different color than females. Usually the colors will be the high school mascot colors.
Decide on a gown size. The sizes will be for height and weight measurements.
Look at the website or catalog of the company the school recommends for you to buy the cap and gown. Many companies offer the choice to rent the cap and gown. This will offer you a slight discount but you will need to return it in good condition.
Look at newspaper classifieds. Since the graduation gown is only worn once, most people want to sell theirs to get money back. If you look at your local ads, you may be able to find the exact color you need. Before purchasing, you may want to ask to look at the gown to be sure it is the same color as your school requires.
Search Craigslist and Ebay if you are unable to find a commencement gown locally. You may want to ask where the gown was purchased. A blue cap and gown may differ from one store to another.
After graduation, sell your cap and gown. If you were unable to get a cheaper cap and gown in the first place, you can at least make money back afterwards. You can sell it through your local newspaper, Craigslist, Ebay, among many alternatives.