I need to hire a new receptionist and medical assistant, and I want to administer a written IQ test or some sort of standardized test that will let me know if the applicant can think logically. Where can I get standardized tests to screen applicants?

You can find some standardized tests online. It's a matter of deciding what you want to really test for - personality, attitude, or IQ.

Here are some sources:

Make sure that whatever test you choose has been validated and can stand up to Equal Employment Opportunity Commission (EEOC) standards. The vendor should be able to certify the validity of the test and be willing to defend it in court on behalf of the practice if necessary. On another note, I am not sure you want to go down the route of the IQ test. It may raise more problems than it is worth. 

You may get a more realistic response by asking the prospective employee real-life, what-would-you-do types of questions. For example, ask potential receptionists what they would do if a patient from your shot clinic passed out in the waiting room after immunotherapy. What if a patient walked in off the street and asked for a prescription refill? Use the sorts of real examples that frustrate you now.

Also ask for examples of how the candidates handled stress well, juggled multiple tasks, or showed empathy. You'll see by their answers if they understand what you mean.