The Alien Blaster is located in the top right hand corner of the map , if you zoom in and count the squares its the 3rd one down and on the border of the 5th and 6th ones in from the right. An easy way to find it is to go due north along the power lines from the MDPL-13 power station (5th square down 6th one in from the right ) the 3 smokestacks make a good landmark  ( dont forget to go inside for another unique weapon the Fisto powerfist and a nuka cola quantum  )  follow the powerlines north keeping them on your left until you see the broken pylon , follow the direction its pointing (the alien ship apparently clipped it when it crashed ) until you start getting radiation poisoning . Next to a small cliff west of the road , just beyond where you can see the power station smokestacks to the south , this weapon will vaporize almost anything in the game with one or two shot make sure you collect all the ammo in the vicinity its very rare you cannot buy it and theres only 2 other places ive found it one is the train station by tenpenny tower the other is the crater with the truck in it by fort bannister you cannot repair the alien blaster yourself but anyone who repairs weapons can repair it for you
another  superweapon thats easy to get is located in girdershade . girdershade is located in the bottom lefthand corner of your pipboy map. if you zoom in till you can see the grid its in the 4th square up from the bottom and the 2nd one in from the left. only 2 people live in girdershade Sierra Petrovita and her  'boyfriend '  Ronald Laren . talk to Sierra and take her tour to activate the Nuka Cola Challenge quest . next talk to Ronald if you win the speech challenge he will pay double for each quantum cola you find( this is useful if you need caps) finish the quest then kill Ronald he carries a unique double barrel shotgun called The Kneecapper which does significantly more damage than a regular one if you dont care about the caps you can just kill him right away . by the way be careful not to use The Kneecapper  to repair a regular shotgun because if you dont save often its gone forever ( i save about every 15 to 20 mins it takes seconds and saves major headaches if you die or F up)

The Reservists Rifle is a unique sniper rifle carried by the Drifter .  You can find him at the Dickerson Tabernacle Chapel (top left hand corner of the map 4 squares down and 4 squares from the left ) . He is sitting in the rafters looking out the front of the building , he also carries the Druids Hood and coordinates to find Oasis which will automatically be added to your map.