Any time I've bought Tahini in the past it's been a puree. I've never seen a powder form. You can order it mail-order if you're really stuck; even Amazon carries various brands now.
As for making hummus, my mom used to make it all the time when I was a kid and while it was thicker than the store-bought variety, it was still delicious. (You can fix the thickness with extra olive oil, she just tried to keep it as healthy as possible.)
Here's a basic recipe from cookinglight.com, but you can search there or at epicurious and find lots of recipes for different varieties of hummus:
3/4 cup water
1/2 cup tahini (sesame-seed paste)
6 garlic cloves, peeled
6 tablespoons fresh lemon juice
1 tablespoon extra-virgin olive oil
1 teaspoon ground cumin
1/2 teaspoon salt
1/4 teaspoon black pepper
2 (19-ounce) cans chickpeas (garbanzo beans), rinsed and drained
Place first 3 ingredients in a food processor; process until smooth. Add juice and remaining ingredients; process until smooth, scraping sides occasionally.
Yield: 4 cups (serving size: 1 tablespoon)
NUTRITION PER SERVING
CALORIES 34(40% from fat); FAT 1.5g (sat 0.2g,mono 0.6g,poly 0.6g); PROTEIN 1.2g; CHOLESTEROL 0.0mg; CALCIUM 8mg; SODIUM 69mg; FIBER 0.9g; IRON 0.3mg; CARBOHYDRATE 4.3g