Mass-effect – Why are quest items not showing up in the Spectre terminal

The "Main Story" assignments are indeed the "Priority: X" assignments, and yes, some of them do branch.

Usually, however, this branching happens on or near the primary planet, such as the Turian Platoon on Tuchanka, or the Geth Server on Rannoch.

Pistol M-77 Paladin:
It can be purchased from Spectre Requisitions terminal in the Spectre Office at the Citadel for 200,000 credits.

The scorpion pistol, available on Salarian Homeworld Sur’Kesh, can fire sticky explosives making it a better choice (depending on your needs).

Shotgun M-11 Wraith:
It can be purchased from Spectre Requisitions terminal in the Spectre Office at the Citadel for 250,000 credits.

The plasma shotgun, available on the Rannoch mission, is probably superior to this weapon.

Sniper Rifle Black Widow:
It can be purchased from Spectre Requisitions terminal in the Spectre Office at the Citadel for 250,000 credits.

This is devastating in the right hands, like a Vanguard on a kill spree, and is worth every credit.