For the pleated filter media, I suggest you look on marinedepot.com. I can't find the category for it, but several pleated filters come up if you search for lifegard.
The carbon can be replaced with any aquarium grade activated carbon, no reason to spend extra on the specific brand stuff. In the brands that are not tied to a specific filter, the premium stuff has less dust, so is worth the extra, but since it comes in bulk, it's usually cheaper than the filter-specific stuff.
Do you have a bioball section? If so, you don't need the undergravel filter. I'm not sure I understand the system, but you might be able to convert one of the pleated media or carbon media sections into bioballs. (Being a plant person, I'm prejudiced against undergravel filters. Go ahead with it if that's your preference.)
Looking forward to the picts, or a description of how these are pumbed together. If they are in sequential order, you can probably modify it to your heart's content. Paralell filters though would need to be kept mostly the way they are.