There are a couple of optional vendors who will sell armor in general and my armors specifically. These vendors will always have at least one of every armor and outfit added by the Mega-Pack in stock with every inventory refresh period.

Roxxi Fallout: New Vegas

Dudley Tale of Two Wastelands

These vendors specialize in unique armors from the Mega-Pack, and are the only legitimate way of obtaining them without console commands. They were split off on their own so that if you don't like certain armor types in your game, or find them immersion-breaking, you can simply choose not to install these vendors and never encounter those types of armor during your playthrough.

"Honest" Herb Fallout: New Vegas (Power Armor)