Fallout 3

Where can I find blueprints?

Helps you build your own weapons.

Fallout.wikia.com/wiki/Fallout_3_crafting that lists them all where they are and what u need to use to make them hope that helps

Or If you have Broken Steel Just get to level 26 and get "Warmonger" perk at that point it's the only useful perk.

Items You need for all weapons(Going from memory so double check the notes section when you get the blueprints)

Dart Gun: RadScorpion Gland, Surgical tubing, paintgun, toy car
Deathclaw Gauntlet: Deathclaw Hand, Wonderglue, Medicalbrace, leather belt
Bottlecap Mine: Lunchbox, cherry bomb, sensor Module, 10 caps
Quantum Grenade: Turpentine Abraxo Cleaner, Tin Can, Nuka Quantum
Shishkabob: Lawn Mower Blade, Motorcycle Gas Tank, Motorcycle Handbrake, Turpentine
Rock-it Launcher: Vacuum, Fire Hoz Nozzle, Leaf Blower.... something else
Railway Rifle: Steam Guage Assembly, Pressure Cooker, two others

The travelling traders usually have one each and there are 4 traders in all who show up at all the major towns, so you can either kill them and take the prints or steal them off the traders, easy blue prints.