I buy it at the grocery store. But if you can't find it you can always make it using cornstarch. I found it on www.ehow.com
just search homemade boiled starch. They have tips too. I have it taped in a cabinet in laundry room:
1 tsp. to 1 Tbsp. cornstarch, 2-3 Tbsp cold water. Dissolve in large measuring cup. Then add boiling water to make 1 cup , stirring constantly. Add cold water to make 2 cups & let cool