Making a Bootable Windows 7 USB without Bootcamp
Microsoft has a tool for creating a bootable USB stick installer that works with Windows 7, 8 and 10. You can find it here. http://www.microsoftstore.com/store/msusa/html/pbPage.Help_Win7_usbdvd_dwnTool#installation

The caveat is that you will need a windows PC to use the utility.

Now Bootcamp will do the same thing plus it will put all the right drivers in the installer to be installed with Windows and help you resize the Mac partition and create a Windows partition. You don't technically have to do it that way though. If you open the Boot Camp package there will be a Windows driver installer package you can copy onto your Windows 7 install USB media and then run once you have windows installed.

Note that you will likely have to re-partition the drive yourself. the Windows 7 installer will not likely resize your Mac partition, you'll probably need one of those Linux bootable "rescue" utilities that will let you resize your Mac's partition so you can install Windows in the partition you could create with it.

It is not terribly difficult but does require enough computer experience to not mess things up and do everything in the right order.

One bit of advice though make a backup image of your Mac's HD before you do anything else. As things going wrong are not unlikely at all.