Sorry to disagree with Niall, but I've never heard of a check valve being referred to as a vacuum breaker. A vacuum breaker could be used for this purpose, but the efficacy would be questionable since, even in the best-case scenario, it would not prevent backflow of the water in between the vacuum breaker and the tee from which the hose is fed, and I don't think the inspector would approve it. Attempting to pass it off as a check valve might even annoy the inspector, and pissing inspectors off is a really bad idea because they will then go over everything with a fine-toothed comb and throw the book at you.
If your water pressure is low or just right, I'd go for the swing-type check valve. These feature a door that is pushed completely out of the way of flowing water, but if water flows in the wrong direction, gravity and the water flow will quickly snap the door shut. (Diagram) Since gravity is crucial to the operation of a swing-type check valve, the orientation of the valve is important. If it is mounted in a horizontal orientation, the hinge of the door must be on top. If it is mounted in a vertical orientation, the intended direction of water flow must be upward. This causes gravity to help shut the door, and it also prevents scenarios where the backflow is very slow, failing to create enough force to overcome the gravity pulling the door open.
If your water pressure is too high, use a spring-type check valve, which features a spring that pushes a ball, forming a seal if there is no water pressure forcing the spring to compress. (Diagram -- water can go up in this diagram, compressing the spring, but if it tries to go down, the ball forms a seal) Since the ball and spring never move out of the way of the water flow, these types of check valves slow the flow of water down.
Almost every hardware/HI store that carries plumbing hardware will have check valves. Both Lowes and HD carry them in multiple sizes and materials, but I don't think Lowes carries the swing-type check valves.
As Smapa has said, it is possible that a check valve already exists in the hose. Look for a bulge in the line that looks something like these pictures (1, 2, 3). If you see something like that, remove the hose and try to blow air in the direction opposite of normal water flow (keep in mind those bits about the orientation for swing-type check valves.) If you can't blow air through, there's a check valve in the hose. Call the inspector and ask if he will approve this if you can demonstrate that it works. If he comes over, be sure to know how to reattach the hose. If it is a gasket connection, you just have to tighten it down (but not too tight, as overtightening can tear the gasket.) If it is threaded, you need PTFE tape (watch a youtube video to learn how to use it) or pipe dope. Don't look like an idiot putting it back together wrong in front of the inspector; see last sentence of the first paragraph.
