Yummie - love hummus!!
I've bought Tahini (the green jar from al wadi) at COOP/Pfauen here in Basel.
So if you have a COOP nearby that has a "large"/largish exotic food section, you might be lucky :-)
A quick search I found this recipe for tahini on another web-site - (but I put in a disclaimer...

I have not tried it and can't say if it is any good):
Tahini (Sesame Seed Paste) Recipe
2 tablespoons sesame seeds
1/2 teaspoon sesame oil
1/4 teaspoon salt
1/4 cup tepid water
Blend sesame seeds in a blender and grind until smooth. Add sesame oil, salt, and then slowly add 1/4 cup of water while blending. Blend until completely smooth.
Yield: 1/2 cup
Note: Sesame seeds turn rancid quickly, so make certain yours are fresh. Be sure to store them away from light and heat and use quickly.