There are a total of 7 weapons that you can create at workbenches. To do so, you must have found at least one (1) of the schematics for that particular weapon. I will list each weapon, where to find the schematics and the materials to actually create the weapons.
1) Dart Gun
Schematic Locations:
MDPL-05 Power Station [LAT -27/LONG 25] - Located on the ground near a skeleton.
Temple of the Union [LAT 13/LONG 15] - Possible reward for completing the quest Head of State.
Tenpenny Tower [LAT -16/LONG -17] - Sold by Lydia Montenegro.
Materials needed to craft:
1x Paint Gun
1x Radscorpion Poison Gland
1x Surgical Tubing
1x Toy Car
2) Rock-It Launcher
Schematic Locations:
Wasteland Mini Encounter - The schematic is sold by Crazy Wolfgang (a traveling merchant).
Vault 101 [LAT -04/LONG -04] - During the quest Trouble on the Homefront, go to the medical area of the Vault. On the wall, look for a framed quote of your mother's favorite bible verse. You need a Lockpick skill of 50 to obtain the contents behind the frame.
Megaton [LAT -01/LONG -06] - Sold by Moira Brown at Craterside Supply.
Rivet City [LAT 18/LONG -17] - In the bridge tower on the third floor. You will need a Lockpick skill of 100 to obtain.
Materials needed to craft:
1x Conductor
1x Firehose Nozzle
1x Leaf Blower
1x Vacuum Cleaner
3) Nuka-Grenade
Schematic Locations:
Wasteland Mini Encounter - Sold by Doc Hoff (a traveling merchant).
Girdershade [LAT -26/LONG -11] - Possible reward for completing the quest The Nuka-Cola Challenge.
Cliffside Cavern [LAT -10/LONG-17] - Inside the Yao Guai Cave. Can be found next to some skeletons.
Materials needed to craft:
1x Abraxo Cleaner
1x Nuka-Cola Quantum
1x Tin Can
1x Turpentine
4) Bottlecap Mine
Schematic Locations:
Little Lamplight [LAT -26/LONG 02] - Sold by Knick Knack.
Jocko's Pop & Gas Stop [LAT -22/LONG -12] - On the work bench inside the shack.
Tenpenny Tower [LAT -16/LONG -17] - Inside Daring Dashwood's safe.
Materials needed to craft:
10x Bottle Caps
1x Cherry Bomb
1x Lunchbox
1x Sensor Module
5) Shishkebab
Schematic Locations:
Wasteland Mini Encounter - Sold by Lucky Harith (a traveling merchant).
Brotherhood Outcast Shack [LAT -14/LONG 25] Clost to SatCom Array NN-03d, it is on a table that is near a work bench.
Meresti Trainyard [LAT -01/LONG 07] - Possible reward for completing the quest Blood Ties.
Materials needed to craft:
1x Lawn Mower Blade
1x Motorcycle Gas Tank
1x Motorcycle Handbrake
1x Pilot Light
6) Deathclaw Gauntlet
Schematic Locations:
Random Mini Encounter - At any given point, you may come across a Wastelander fighting a Deathclaw. Be ready to finish off the Deathclaw because he may attack you! Once you are safe, go to the body of the Wastelander and loot the schematic.
F. Scott Key Trail & Campground [LAT -27/LONG -15] - Inside the caravan/camper in the picnic area. (Be cautious...a Deathclaw is roaming around the area!)
Rivet City [LAT 18/LONG -17] - Possible reward from Bannon for completing the Freeform quest Council Seat.
Materials needed to craft:
1x Deathclaw Hand
1x Leather Belt
1x Medical Brace
1x Wonderglue
7) Railway Rifle
Schematic Locations:
MDPL-13 Power Station [LAT 02/LONG 17] - Inside the building on the work bench.
Rivet City [LAT 18/LONG -17] - Possible reward from Abraham Washington for completing the quest Stealing Independence.
Museum of History Entrance - Sold by Tulip in Underworld.
Materials needed to craft:
1x Crutch
1x Fission Battery
1x Pressure Cooker
1x Steam Gauge Assembly