There are four ways to find new gear, such as weapons, armour, harness and whatever might help survivors.
Established by military, they cover bunkers, checkpoints and command posts across the city. Outposts will have military gear available to player. They will either be made available by Solider Players rank or as rewards for completing missions given. Needless to say dead military personal also have these stuff.
Long list of gear available at outposts be here.
Those are the gear found on corpses or bartered from people that have survived the initial waves that devasted London. Some of them are better then military gear while some simply are subpar. Enthusiasts cover everyone interested in such stuff. A gang member might have some stashed some ammo and a gun. Or a local gentlemens club might be keeping a boomstick just in case.
Long List of enthusiast gear be here.
Like wise Police gear is available on less then fortunate policemen across the London. These gear tend to be easier to use then Military gear and come with already equipped with accesories. They are also looted by more agressive survivors. They in Diablo 2 munchkin language "Enchanted" loot.
Long List of police gear be here.
She will open shop in some random safe house in the mission area. She accepts cash only and won't buy used stuff. Her inventory covers everything (including modded weapons and kits). Prices should be so steep that players should pool money get exactly what they want. Players can place an order too. Which means Players can come up with a weapon that is not listed, its up to to DM accept the weapon, tinker with it or out right refuse. Once an order is placed Trader will go get the requested weapon for an obnoxious price.
Even longer list of stuff.