Creating a Bootable USB Drive: unable to download Mavericks again
You can not download Maverics Installer if you already have mavericks.

Next time when you download a major new OS, DO NOT RUN after download.

First find the download and make copy the Installer file.

The reason is the Installer will delete it self after installation is complete.

How to make a Bootable USB for Mavericks has been discussed and answered here.

Last not least: There is a Recovery (hidden) partition on you drive.

When called upon that one will recover (repair) your mavericks from the Internet. The recovery can be also done from a USB. as explained here, it only needs a small USB with 1 gig size.