You will have to check around. Availability and prices vary greatly depending on where you live. In some places you wont be able to buy the glass locally for cheaper than the kits cost. Other places you can get good, cheap, accurately pre-cut glass at Lowes/Home Depot/Ace. Where I live only Ace carries or cuts glass.
Also, if you decide to call a glass shop, compare prices as they will vary greatly as well. For instance, I had some glass lids cut for 10 gallon tanks and was quoted prices from 2$ a pane to 12$ a pane and everywhere in between.