Is this what you were looking at? Can you tell us what crankset you have, or post a picture?

Also, a) you probably have a freewheel, not a cassette; b) what are the cog teeth numbers on it?

Finally, have you been riding long? Sometimes beginning riders think they need lower gears, then discover that the initial part of the development curve is pretty steep. Ie. you might find that you get into better shape pretty quickly and your current setup is adequate.