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.
I also would not recommend Lowe's/Home Depot. Around me there is only one Lowe's that cuts glass, and it only has one thickness, which is too thin for vivs. A lot of times the people who cut your glass there don't know what they're doing also...
It seems easier to buy kits unless you have a local guy to cut for you (everyone seems to hate lowes). I did have a local guy but it was so expensive. The only issue with buying kits is the time waiting for them as well as shipping cost. I guess i will look around and check my local prices.
I have had glass cut at lowes but be warned they dont always cut it as accurate as you may like and in turn it may not fit. Also a down side to having lowes cut glass is its not very thick and is not so much fragile but is alot thinner then i like. Plus the edges of the glass are sharp and notorious for cutting you, learned that the hard way! I just had my local glass shop cut my glass for my 46 bowfront and if I would have known they would have cut all my glass, its alot thicker glass 1/4in plus they smoothed down the edges of the glass to insure you wouldnt cut yourself! I would recommend a local shop over lowes any day
My local glass shop charge me 20 bucks for a 36"x18" size glass (1/8th) and they do all the sanding on the edges, so you wont get cut. In top of that, they do any other cuts on the glass for free. I need mine cut down the middle so i can make a living hinge for my horizontal tank. If i ever do a vertical build, i would follow the DIY i just posted. It looks awesome.