where can i get schematics for a dart gun in fallout 3?

Schematics - Dart Gun are the schematics needed to make the Small Guns weapon "Dart Gun". Possessing multiple copies of schematics increases the initial repair condition of the weapon: One copy gives a condition percent equal to 75% of the player's repair skill, two nets 100%, and three nets 125%. A schematic can be obtained from Hannibal Hamlin for completing the Head of State quest for the slaves.
A schematic is at the Boutique Le Chic in Tenpenny Tower and if the player sides with ghouls during the Tenpenny Tower (quest), it can be purchased twice: once from Lydia Montenegro and again from Michael Masters, the ghoul who takes over. You can actually get both for free. The second Tenpenny Tower Dart Gun schematic obtained in this manner will behave like a normal schematic and upgrade the quality of your future dart guns. If you use this method to obtain three schematics (one from some other location, and two from Tenpenny Tower), and then discover the last remaining schematic, the last schematic will not act as a regular schematic and will merely remain in your inventory in the Misc section.
A schematic can be found at the MDPL-05 Power Station next to the skeleton of a dead engineer.