So before you even start, you'll need a tailor (preferably Mooncloth specced), leveled all the way up to 375. There are a few of those around, and if your guild is raiding endgame, you should have at least one, so that's not too hard. But to make this bag (the pattern is available from the trainer in the Lower City of Shattrath), you'll need four Netherweb Spider Silk and eight (yes, eight) Primal Mooncloth.
The spider silk ain't no thing-- you can grab it from the spiders in Terrokar or even the spiders in the animal wing of Karazhan if you want. But the Mooncloth is the kiler. For each Mooncloth, your tailor has to have one bolt of Imbued Netherweave, one Primal Life, and one Primal Water. For a bolt of Imbued Netherweave, you need 3 Netherweave bolts and two Arcane Dusts (available from an Enchanter). And for a bolt of Netherweave, you need 6 Netherweave cloths.
But oh wait, there's more. For a Primal Life and a Primal Water, you need 10 each of Motes of Life and Water (that drop from plants and earth elementals in Zangarmarsh, Nagrand, and elsewhere). All total, then, you'll need 144 Netherweave cloths, 16 Arcane Dusts, and 80 Motes each of Life and Water. Not anywhere near impossible, but that's a good few hours of farming, for sure. A Mooncloth tailor produces two Mooncloths per click, so with an MC tailor you can actually half those mats. But that's still a lot to find.
And then there's the cooldowns. Mooncloth making actually has a four day cooldown on it-- each tailor can only make one Mooncloth (or two if specced right) every four days. So not only do you have to wait, but if your tailor is backed up with other orders, it could take even longer than two weeks to get your Mooncloth from them. Of course there's ways around this (use four different tailors and you can get it done faster), but getting some tailors to work for you is harder than farming those Motes sometimes.
Enough work for you? But farm all that stuff, hook up with a tailor (or four), and the 20 slot bag can be yours. Now for the other three bag slots...
Getting Rid of It: Are you crazy? After all that hard work? These are BoE, so they can sometimes be seen on the AH, for up to (and upwards of) 500-1000g. Otherwise, if you've already used it, and happen to have all your bag slots full of these, a vendor will give you 5g for it. What a waste.