Well, how much money ARE you willing to spend? There are basically two types of shotgun (or boom) mics: electret condenser and “true” condenser. Electret condensers are the same mics you find on ENG cameras. They’re basically fine for picking up general ambient sound, but even putting them on a boom isn’t going to get you the sound you’re looking for (unless the mic is no more than two feet away, all the time, and there isn’t any other background sound going on). They start at around $250. True condensers can cost four times that much but, they can also “hear” about four times as good. They also reject background sound much better.
Now, all that being said, and considering your budget, I would recommend the Audio-Technica 4073a. It’s the least expensive of the “true” condensers (at around $549) and, unlike a cheap electret, you can always re-sell it for about what you paid for it on Ebay. I personally have never used one but, I’ve heard good things about it. Otherwise you’re looking at spending upwards of $1,000, and that’s without a windscreen! Of course you could always rent one :o)