Where & When Did The Courier Get All of These Items From? (Courier's Stash Add-on)

Buying it from merchants.

 

Josh Sawyer hated the idea of preorder DLCs, it was something Obsidian made them do, and when New Vegas came out he made a mod for the game rebalancing it to make it a bit tougher known as the JSawyer mod. It’s a mod that’s very popular to this day. One of the things his mod changes is that you don’t get the preorder DLC packs instantaneously like you do in the base game, but rather you find it spread across the map as you play the game. For example the Classic Pack with the Armored Vault 13 Jumpsuit, Vault 13 Flask, and Weathered 10mm Pistol is found in the trunk of the Crashed Highwayman Wreckage.

Now I’m not gonna debate anyone on the canonicity of mods, but if the lead project director for the game makes these changes to better reflect what he had in mind for the game, I consider that canon. So the answer to your question is that the Courier happens upon these loot stashes across the Mojave as they venture.

 

It's also worth noting that, originally, the Courier's Stash load outs were not given to every player. Rather, specific games retailers who you pre-ordered from, each had a different one of the four load outs. The only company I remember that participated was GameStop, don't remember which outfit came with their version of the pre-order. But you get the idea, a retailer specific pre-order bonus, where it would only be possible to get one of the four.

Then after the game and most of the DLC had shipped, the four outfits were packaged together as a a horse armor tier cash grab.

The important thing to take away here is that, from a canon perspective, I don't think we should assume that any courier actually has more than one of these outfits. There's a version of the Courier with the Vault 13 get up, a tribal Courier, a veteran caravaneer Courier, and a mercenary Courier, each of these help you to role play. But I don't think that there was any courier carrying all of these get ups around with him.

As to your question of how the Courier would ever get their hands on Vault 13 gear - hard to say, but at the same time there's any number of possibilities. Maybe your Courier is actually a descendant of the Chosen One (I have a feeling this is what they were going for at least as a possibility, given that the last West Coast game had you as a direct descendant of the first game's protagonist). Maybe your Courier ran into an antiquities dealer that was selling Vault 13 gear unrelated to the Chosen One or the Vault Dweller (there would have been a lot of gear in Vault 13, and it doubtless would have been valuable to those who found it after the Deathclaws were massacred).

 

I remember preordering from Walmart just so I could get the caravan pack

 

This.

I dont think the V13 stuff is original. More like replica made by NCR merchants, what with Vault 13 being religious and fashionable to them.

A courier going one way toward NCR can gather a full suit of caravan guard and/or tribal package. Return from NCR will get them a complete package of mercenary equipments and V13 souvenirs.