At big box retailers, you can buy 10-foot sections of gutter. They are cheap, usually a little scratched up at the big box 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-foot sections require splice seams in the middle of the runs. That is a problem because, over the years, they will leak. It’s also 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!