There are a total of 9 different weapon manufacturers in the game that you can obtain a loyalty package from. These will be in your mailbox, under the "social" tab on the start/pause menu. Every 100 kills with a gun from a certain manufacturer, you will get a different letter/package from that specific manufacturer/company. The manufacturers that you need for this achievement are:
Atlas
COV
Dahl
Hyperion
Jakobs
Maliwan
Tediore
Torgue
Vladorf
It's recommended to keep a list handy of which manufacturers you have left. Once you open your mail and receive the gun they manufacturer sent you -- the letter will disappear and there is no way to look at your read mail. The good news is that you will get a new letter/gun for EVERY 100 kills you've done with guns from that certain company. So if you're not sure about a company, kill enemies with a gun from that company until you get mail and then move on to the next manufacturer.