Where can you buy backpacks?

[Cat]

Aside from the starting one, I haven't seen a single one all game, is crafting them the only option now or does someone sell them? Having one backpack to share across the party is a bit silly.

[Tremualin]

I got backpacks really late in the game, almost at the end, from high level enemies and high level faction vendors and seventh heaven vendors.

I had the perk that gives you 15% more rare items. Didn't seem to add backpacks to regular vendors.

[iriemk]

Nab the backpack they threw in one of the DLC's loot (no rubles, but euros/dollars required to get that^^), Then go get Powerarmor, then go 2 Underground Test Site, final fight nets at least one satchel. Plus the fully upgraded Powerarmor with, what, +120 carry cap - your whole party is almost set, since you cannot equip a pack with the exo-thing anyway.

EDIT: You can shwack the Steel Shield Mercs @ Mikhaylich Tavern to get 1 backpack, but you have to deal with them during the start of each faction questline either way, so this one's difficult to miss.
Another one drops during the final confrontation @ Mutant Underground, if you shwack the Red Guards. Dunno if any of the Muties drops one if you decide to go with Red Guard.

Otherwise, if you wanna purchase your packs, the only merch who sells proper backpacks (ERMACs) more or less regularly , is the gunsmith @ 7th Heaven...
Apart from that, they are a rare sight in all shops that sometimes have armor in stock, but I'll guess that you'd have better luck finding a pack at a vendor that regularly sells armor, unlike the faction vendors or blackmarket, who only rarely sell a piece. The Smith @ Police HQ is probably your best bet.