Anna: This is my recommendation.
At least one person from your facility should be trained via a BRC course. What you can do is then bring back all of the knowledge you gained and train the rest of your internal audit team.
Internal Audits are critical to your success and your going to need to show competency in this role either way. On top of that, Internal Audits are a huge undertaking (if done correctly) and it would be in your best interest to take an official course.
You do not HAVE to take the course, but it's highly recommended and your auditor is going to want you to prove that your knowledgeable of these types of audits.