There are a number of injection gizmos on the market ranging from simple hypodermics to pumps that look like something used by the Orkin man.
For home use, a good sturdy specialty meat injector hypodermic will do. The needles for this purpose are different than normal hypodermics. They aren't open at the tip because a large opening at the tip gets clogged easily. Meat injectors have holes in the sides of the needles, and the tip is a sharp point.
When you inject it is important that the tip go all the way through the meat until it almost pops out. If the needle isn't long enough then go halfway through, flip it over and inject from the other side. Inject every inch or two an, this is important, as you withdraw the needle, continue to push down on the plunger so injection is dispersed from top to bottom.
A good injector has a really sharp tip, and a sturdy connection between the needle and the body of the syringe, but the needle should be easy to remove. The plunger should have a sturdy connection to the body and a good tight gasket between it and the interior of the syringe. I prefer a silicone gasket. It should be easy to break down and clean, and you should be able to store the needle inside the syringe so it is compact and you don't lose the needle. It should have a capacity of at least two ounces and be made of stainless steel. The inexpensive plastic syringes I've owned eventually cracked with age or burst under pressure. Brass, copper, and aluminum are not good for this purpose, since they can react with the salt. Look for stainless steel.
The problem everyone has with injectors is filling them. Most of us mix the injection and stick the needle into it and suck it up. But the position of the holes in the needle prevents you from getting it all, and they suck up a lot of air. This can be aggravating when you are using just a half stick of butter for a turkey breast or a small amount for a couple of turkey breasts.
The solution is to mix your injection, pour it in a glass or cup, and put a drinking straw over the needle. Stick the straw in the cup and it works perfectly!