This is the real deal, the horseradish used to make Japanese Wasabi. Not for the beginner, but the rewards are well worth it. 
 
You will want to duplicate the conditions that these are grown in, in Japan, as closely as possible. 
WASABI SEEDS NEED A PERIOD OF COLD, SO THEY ARE BEST STARTED IN THE FALL. You can simulate these conditions by cold stratifying in your refrigerator, for 60 days. Seeds should be soaked the night before planting and planted in flats, with the flats placed in sealed plastic bags and placed in your refrigerator. 
 
Parts of the Pacific NW and Blue Ridge Mountains have the perfect conditions for growing wasabi, as they prefer temperate and humid conditions. If you do not live in these areas, you can re-create the environment in a greenhouse, just be sure to adhere to the temperatures below, they must not fall or rise out of this range.

Wasabi needs to be grown in temperatures between 45 and 70 degrees
Wasabi grows naturally under forest canopies and gets no direct sunlight. You want to ensure that the plants are well shaded with filtered light.

They need rich soil with organic compost and sulfur rich fertilizer
Wasabi likes to be kept moist, but not muddy and waterlogged. To check whether the soil drains well enough, water the area well and watch the water soak in. If it's slow to be absorbed, work in more compost. If it drains right away, the soil is fine for wasabi.

Planting wasabi near a natural pond or stream is a good idea, since the soil will stay constantly moist, but will naturally also drain well.

You could also plant wasabi near a waterfall that will continuously splash on the plant to provide water. THIS IS A SEMI-AQUATIC PLANT. If you do not have such a water source, mist the plants daily with a hand mist sprayer to prevent wilt. Do not, however, soak them which can cause rot. 
Harvest takes 24 months. 
 
Growing zones 8-10 (Please note that not all zones are created equal. I am in zone 10 (Florida) but would not be able to grow these outdoors. Zone 10 in the Pacific NW is completely different and ideal. If you are unsure of whether your zone would be appropriate, go by the temperature requirements, which are between 45 and 70.)
 
These seeds (and sweetgrass, are the only seeds in my shop that I cannot guarantee, because there are so many requirements that must be met.)
 
This is for 10 seeds