Michael can be found in Sanctuary. His location changes randomly, so you'll have to do a bit of searching. All you need to do is simple talk to him and he'll hand you a weapon. Once you've accepted it, the achievement will unlock. Below are a list of his known locations.

Scooters Garage
Marcus's Shop
Centre of map, next to the "This Just In" vendor
Moxxxi's, at the bar or next to the piano & jukebox
Outside the Black Market
Crimson Raiders HQ
Down by the Sheriffs near the Dhal garbage
Dr Zeds
Stairs near Claptrap