On one of the wiki's I found a reference stating that it is possible to enchant "Fortify Carry Weight" on a few items.
I am currently trying to track down a disenchantable item that has this enchantment so I can apply it to my boots and gloves (I ran out of useful enchantments), so far though I haven't had much luck.

Does anyone know if there is a location with such an item or should I assume that the drop is completely random and/or none existent?

I was unaware there was one. Idk. It's probably random. Get the extra pocket perk.

Totally random bud.

I found Leather Boots with the enchantment for 30 pounds. They were in a random loot chest.

I can't think of a fixed item with that enchant that's disenchantable

Check your favorite stores often!

Okay, thanks everyone, I'll just keep exploring then.

I just bought a +30 pair of Scale Boots from the Khajit caravan when they were set up outside of Markath.

Yea I found dwarven boots of fortify weight of 50 pounds. When I picked them up it said bring them to calcermo in markarth. Try talking to him to see if he ask you to find them for him?

Screw Calcermo, keep the Boots. The ones I bought cost 2880 gold while wearing the Thieves Guild Hood.

I got both lol. When I went to him he said hand them over I said I'm not ready yet so I went and disenchanted them but when I went back to him I was still allowed to give him the boots that I no longer owned. He paid me like 1000 coin for them too. I guess it's a glitch But hey it works

Go to Sinderion's Field Laboratory from Black Reach, and next to the chest to the left is a pair of boots with 30 fortify carry weight.

It seems to me I've found "Hauler's Boots" in Dwemer ruins as random (or not) loot, more often than other places. I don't think I've ever seen them in a store so far.

Just join the Theives guild and after you do the first quest u get a thieves leather armor set, on of which has the enchant to carry extra weight!

I was given a pair of "Iron Boots of Lifting" in Whiterun, for doing the Dragonstone quest. Guess I got lucky.

Got the same and immediately disenchanted them, but so far I find that fortify carry weight potions are more useful than the enchantment. That may change once my enchanting skill goes up.

I have done a fast travel circuit from Markarth -> Whiterun -> Riften -> Windhelm -> Solitude -> back to Markarth and have yet to see them. I have done this trip at least 15 times now trying to power level enchanting/smithing/alchemy. I have even seen the banish enchant about 10 times. I think you can only get them out of chests or off mobs.

EDIT: LOL; on the 16th trip I found a pair of Dwarven boots with +35 carrying capacity from Oengul War-Anvil from Windhelm. Right after I posted this response. I guess vendors do sell them; they are just extremely rare...

Just now I was doing that mage quest in Mzfult, toward the end of the dungeon in the "boilery" section I opened a chest with Elven Boots of Brawn which give plus 40 carrying capacity, along with it were some boots with plus 60 stamina.