How can I maximize Wasabi flavor?
Prepared wasabi that you can buy in a common grocery store (both powdered and paste) is basically just horseradish with green dye in it. Really, you've acquired a taste for horseradish. Not that there's anything wrong with that! I would recommend seeking out other horseradish-based sauces and condiments. You'll probably enjoy them better than the fake wasabi.

If you're interested in finding real wasabi, you'll need to look for it in a specialty grocery store (or online). It's traditionally prepared by grating the fresh root on a sharkskin grater. However, what you had with your sushi was most likely the fake stuff, unless you were in Japan or a very high-end sushi restaurant here in the states.