Didn't he say Justin Hammer made it for him? Also, he probably has millions, it's not like he's a common criminal. You could say the same thing about the Judas bullet since that can probably kill Iron Man.
On top of that, at the beginning of Age of Ultron, we see some Hydra goons in armor, flying around, shooting lasers. Granted, the Avengers turned them into canon fodder anyway, but they had them.
There's enhanced stuff all over the place now. Hammer Industries is already established as making this stuff. Remember Iron Man 2? I mean, that's why the world NEEDS superheroes now.
There was a similar suit in Agents of SHIELD, used by the old HYDRA guy who was helping Hive, but it wasn't quite as powerful. Plus, that guy was a senator and a leader of HYDRA, it made sense for him to have the connections to get that kind of tech, but Diamondback- of all people- getting his hands on it, was pretty poor writing. They could've linked Diamondback to HYDRA, AIM, Hammer Tech or Killian Tech instead of him just being a mobster. He's no richer than Kingpin, so the fact one got the suit and the other didn't was just so Luke can actually have someone to fight.
Actually well don't always gotta play into connects....Diamonds in the smuggling weapons business, so all he needs his connect and some startup cash to the weapons he need, plus he could be buying faulty tech. You know, how they sell a product at half price cause tags are missing, buttons place in the wrong place etc etc. He could be getting the tech like that. He also could be buying old military tech, that's new to the street, so he gets a sweet discount price. There's a bunch of way's, he could get the stuff he needs. The issue isn't the how, as much as the why? ....Why should i believe this character can get these tech, and that's where they messed up. Diamondback, just comes off as a loose cannon, that can't pull of the business deals, Shades and the show tells you he can. Like my New song I'm Buying https://www.youtube.com/watch?v=DZ0EfQ3d3Pg
How can people even be surprised by this? I know this wasn't ever explained in the movies since Marvel Studios was just starting the Marvel universe, and they didn't really have everything 100% planned out. But as it is no one should ever expect these plot elements to be referenced in any of the Marvel movies. Marvel Studios claims the movies and shows are as whole, but it's obvious they don't want anything to do with the shows. The movies never even reference the Netflix show or Agents Of Shield.