How do I get a Windows ISO to set up Boot Camp?
You can do this using macs built in DiskUtility.app.

Insert the Windows installation DVD into the external Superdrive.

Open Disk Utility, Select the USB optical drive in the left DiskUtility window. 

Click new image in the top bar.  

Select DVD/CD master from the "Image Format" submenu.  Choose "None" for encryption.

After the image is done, locate it in Finder and highlight the file.  Press enter/return and rename the disk image file extension to .iso

Eject the Window installation DVD and insert your USB that is FAT32 formatted and use Master Boot Record for the partitioning scheme.

Open up Boot Camp Assistant and make sure "Create a Window 7 or later version install disk" is checked.  When you click continue it will prompt you to select the location of your .iso file you created.

Link to Apples Guide for creating a Windows .iso for BootCamp