There are lots of options, and its worthwhile spending some time thinking the purpose of the assessment in order to determine the right measure. However, as you review different measures, I would look at the Beck Anxiety Inventory (BAI). It is proprietary (need to pay to use it), is brief (20 items) and you can get it from The Psychological Corporation. There are lots of validation studies of the BAI, just run a lit search on it to pull the references.
Best of luck.