They will tell you their cables are propitiory, but I have used several USB cables on my apex, and all work just fine. As long as your going apex module to apex, you should be fine. The problem lies with USB being able to carry power along with data. Neptunes cables only supply data, no power. So unless you plug the USB cable into say your PC, then a neptune module, you should be fine.
Look it up in the Comprehensive Reference Manual for the best clarity. The cables are standard USB, but the USB/Aquabus devices are not compatible. The CRM also has a listing of vendors that can make extra long cables without repeaters.
Any USB cable will work as long as it dosent have a active extender. So any normal usb cable. The problem is that if you need more than 5 meters (i think) most usb cables have an extender. This will cause issues with the apex. That is why you can build your own using a cat5 cable and usb connectors.
Just an update on this thread, since I know this question has circulated for some time, and I have been looking thru this issue.
As people have mentioned, USB cables over ~15' are usually active, which is a no-no from Neptune. However, I have found 'passive' USB cables that are >15ft long. They are made by "Comprehensive", come in various lengths, and I have verified with the company that they are indeed passive (item numbers end in ST)
Buy Comprehensive USB 2.0 Type-A Male to USB Type-A Male Cable (25') featuring Transfer Rates up to 480 Mb/s, Compatible with USB 1.0, 1.1, and 2.0, Molded Strain Relief, RoHS Compliant. Review Comprehensive null
www.bhphotovideo.com
They are MUCH cheaper than getting an "official" branded cable and are the exact same thing.