If you have already got Mavericks before, you can redownload the full installer (which will now come as version 10.9.5) from the purchases tab in the mac app store, and then follow instructions here to make the bootable usb:
https://forums.macrumors.com/posts/18081307/
If you don't have mavericks in the purchases tab in your mac app store, it will be more difficult to obtain it.