Unfortunately, Mavericks is no longer available. Apple will not sell a copy at this time - unless you've had Mavericks installed previously, you won't be able to download it.
And, a more important consideration: no Mac can boot from an OS older than what it came with. So if your new one came with Yosemite, it won't be able to boot from Mavericks.
And, if you do see it being sold anywhere (except the app store), then they are pirated/illegal copies and you will have problems because any app store downloads are tied to the Apple ID and password that was used to obtain them - in this case, the seller. You would never be able to reinstall the OS or update apps as you'd be prompted for that seller's Apple ID and password.