Bowfishing reels come in many different styles.
Some kinds of reels screw into the stabilzer hole on your bow. If you have a recurve, some can also tape onto it. This kind of reel doesn't have any way to retrieve the line, so you have to grab the line and put it back on the reel by hand. These don't come with line, and usually cost $10-20.
You could also use a normal fishing reel. Just make sure it has strong enough line (i'd use 50+ pound test for smaller fish). To attach it to your bow, you need a reel seat. A reel seat screws into the stabilizer hole, and the reel attaches to that like a fishing pole. The Zebco 808 is specially made for bowfishing, and costs about 30 bucks.
AMS makes a retriever reel that acts like a fishing reel. It has no bail to release, so the line won't snap if you forget. To reel in however, you must pull a trigger and then reel in. This can be hard to get used to because to much pressure or too little pressure on the trigger will stop line from going back into the barrel it is stored in. It usually runs between 50-60 dollars, but it's very convienient. It comes with 130 lb. test line.