Hey Conker--
I love my ILBE assault pack, but I've always ended up passing on the main pack. I don't have any experience with it, but if all you want it for is camping, there are MUCH lighter options out there.
That said, I collect all things MARPAT. Can't get enough. It's a real problem.
In my experience, eBay is by far your best option for all of it. The Source USMC WXP 3L Tactical hydro system can be had on a few sites, but for an enormous premium. For example,
www.optactical.com has the 3L Tactical in coyote for $54, but if you want the USMC 3L Tactical, it'll set you back $97. You might also see if Source has an online store, but again just keep checking eBay; I eventually scored a used one for $20, plus the cost of a new bite valve at MEC.
There are two "sustainment" pouches, but they're not for the standard ILBE pack system. They're for the Recon ILBE, and I'm not sure what they're called exactly. The PMICE pdf on the Recon ILBE refers to them simply as "supplemental pouches." I think you can get them commercially, but not in MARPAT. Here's a few for sale; you can see the Recon main and assault packs too:
http://cgi.ebay.com/NEW-2-x-USMC-REC...13649827635870
Since these are preposterously expensive, you might consider some of the AITES 200rd SAW pouches that are currently flooding eBay, or a MOLLE sustainment pouch in coyote. They're not too hard to find.
As to the radio pouch, I got mine thrown in with a Gen 1 MOLLE assault pack and a pile of pouches I bought for a friend on eBay. Keep checking and you'll find one for cheap. Likewise with the ILBE waist belt. Can't help you on the pack cover;
www.us-elitegear.com used to carry a lot of this stuff, but they're scaling back their inventory big time.
Hope that helps...enjoy the assault pack!