Second hand is cheaper, downsides are that it takes ages (sometimes never) to find what you're looking for, there's no guarantees about quality, and you'll most likely end up compromising one way or another.
The answer really depends what you're looking for, if it is just a standard 8x2x2' you'll most likely be able to find one. If you're after something wider (8x3 or 8x4) you will probably be looking longer. Do you want it drilled? What sort of stand do you want? There's a heck of a lot to consider with a tank this size, and although a new tank may be more expensive, having everything done the way that suits you for the next decade or two (or however long you plan on having the tank) will often make that well worth the extra cash.
DIY is certainly worth considering too.