I think you know good gyokuro from FuJian in China.
If you want gain real gyokuro you can go to FuJian or ask a friend buy it form FuJian.
As to brew the gyokuro tea,I can tell you something.
Gyokuro is not brewed like the normal sencha green tea you may normally encounter. If brewed like sencha, although it may taste "okay", it will taste completely different than it should. The brewing temperature, brew time, and proportion of tea to water are quite different. Knowing this up front makes a world of difference.
Water quality: Good quality water is essential for making the perfect cup.This does *not* necessarily mean absolutely pure water with no minerals, but rather something a bit better than tap water. Minerals in the water can actually enhance the flavor of green teas.
Water temperature: This is where most mistakes are made. The proper brewing temperature for gyokuro is between 122 F ~ 140 F ( 50C ~ 60C) degrees. This is quite a bit lower than sencha. Will 10 degrees make a difference? Yes! The problem is, if you pour a moderately hot gyokuro into cold cups, your finished tea will be lukewarm. This is why it's important to pre-heating both your cups and teapot. A good proceedure is to pour boiled water that has been given a few minutes to cool off a bit directly into the teapot first, allowing it to sit for a minute or two. From there into the teacups, with the remaing hot water tossed. After everything has warmed up (cups and teapot) and at the same time the water has cooled down to around 130 F ( 55 C), we then add the leaves into the teapot followed by the hot water remaining in the cups. . You can also pour back and forth from one container to another, or simply wait. Use a thermometer and practice until you perfect your technique!
Proportion of tea to water: Another area where mistakes are often made. The proper proportion is 2 tablespoons to 4 - 5 ounces of water (120 ml). This may seem like overkill, but provided you have the correct brew time and temperature, it is not. Since gyokuro is expensive, we can appreciate your concern. However, you can make quite a few cups of tea with this by re-brewing with the same leaves numerous times.
Brew time: Two minutes to three minutes. Will 30 seconds make a difference? Yes! Do not stir, shake, or mix the tea while it's brewing. You can swirl it around a bit as you are pouring it out, however.
Allowing the green tea leaves room to expand: Not to be overlooked! Japanese green tea is compact, and it needs room to expand. This is especially so with gyokuro. Tea balls do not work well! You'll be much better off using either a Japanese or Chinese teapot that was designed with green tea in mind. Here is an example of a perfect teapot for brewing gyokuro: Dragonfly Teapot