Hey, so RAW say that to find an item the player needs to make a Negotiation (Streetwise for black market) check with a difficulty based on rarity to find an item, with a relevant Knowledge skill as a possible substitute. Obviously, Knowledge Underworld would work for Black Market, but for above the board shopping, what do you think might be justifiably? I've got one player with Knowledge Warfare, I think if he had a background of quartermaster that maybe that knowledge could be argued for finding weapons, mods, and armor. He is just a standard grunt though. Am I being a little harsh, do you think that in this circumstance, that I should just let it slide if it fits narratively? Are there any other knowledge skills you all think might be appropriate in general?

Not everything needs a substitute. In the case of looking for military tech (whether legal or no), Knowledge (Warfare) would make sense. And I do mean military tech, not just "weapons."
Blaster Pistol? No. Light Repeating Blaster? Yes.

I do think you are being a bit harsh. You don't have to have a particular background in order to use the skill, otherwise it'd be a talent not a suggested alternative.

Underworld, Education, Warfare, and maybe Lore are the Knowledge skills that would be easiest to use for substitutes. If an item makes sense for the skill, I say let your players roll that Knowledge skill. But not everything will fit neatly into a Knowledge skill, and you shouldn't stretch the criteria too broadly. Negotiation should be used for the majority of checks.

I'd say that as a rule of thumb, the more expensive or rare an item is, the more likely it is to fit with a Knowledge skill.

If your player has a “standard grunt” with a high enough Intelligence/ Warfare to make that much of a difference... yeah, I’d let them use it.