If anyone at ArtMaterials asks if they can help you, hit them with whatever is in your hand and run. If the police catch you, the litigation and eventual time served will be less than the time it takes them to find what you're looking for.
Price check: 18" T-square. Retails for about $10 in other stores. After spending 10 minutes searching through a database and not finding the one I had asked about, he said it was probably $20.