At big box retailers, you can buy 10′ sections of gutter. They are cheap, usually a little scratched up at the big boxes stores, but they work. The problem with gutter sections like this, in addition to the fact that the big boxes only carry an inventory of about half the accessory parts you need to finish the job, is that the short 10′ sections require splice seams in teh middle of the runs. That is a problem because over the years they will leak and it is more expensive to install.
An alternative to the big box stores is to use a seamless gutter roller. This machine can make gutters as long as your roll of coil stock!