Crafting
One of the most effective ways to get high-end weapons is through crafting. Why is it the most effective? Because as long as you have the materials to build it, there is no extra cost whatsoever. Materials are easy enough to come by as you can deconstruct your old items and turn them into weapon parts, fabric, or tools depending on what the item is.
Once you’ve got these basic materials you want to head over to the Crafting Station at your Base of Operations and scroll all the way to the bottom. You’ll then see that you can crafting Specialized and High-End versions of these materials by using multiple materials of a lower quality. For example, five normal Weapon Parts will turn into one Specialized part etc.
Deconstruct as much stuff as you can and keep as many High-End materials as possible. Next you need to go hunting for blueprints. These can be found around the world of Manhattan either in loot crates or dropped by powerful enemies. When you’ve found one, head back to your Crafting Station and simply hold down X on PS4 or A on the Xbox One on the High-End item, and voila! You just crafted your very own High-End weapon.