The Inheritance MainHand Weapon Construction kit is a reward for completing chapter 1 of your class quest. You can redeem it on the legacy vendor in your capital city, but be sure to mail the KIT to the character you want to get it first. Here’s a list of all Inheritance MainHand Weapons.

First, open your in-game mail, if you’ve completed chapter 1 you will have mail waiting for you with the Inheritance Mainhand Construction Kit.

Next, if you want an alt to redeem the item, be sure to mail the KIT to your alt (not the final item). Read Bioware’s clarification on this. To see what items you can buy, visit the legacy vendors in your capital city. Here’s the map for Dromund Kaas.

Next, if you’d like other inheritance pieces, you can get them in 2 different places. The starship upgrade vendor sells construction kits for the gloves (2,000 fleet commendations), bracers (1,000 fleet commendations), and belt (1,000 fleet commendations). For example,

• Agent’s Inheritance Gloves
• Agent’s Inheritance Bracers
• Agent’s Inheritance Belt

The Daily Commendation vendors on several planets sell construction kits for the birthright head (250 commendations), birthright legs (200 commendations), inheritance chest (200 commendations), and inheritance boots (100 commendations).

• Agent’s Birthright Headgear
• Agent’s Birthright Pants
• Agent’s Inheritance Jacket
• Agent’s Inheritance Boots

I haven’t found the sources for most of the birthright construction kits, or the legs / head / offhand inheritance gear.