Yes, immunizations are on your own dime. However, depending on your insurance, most, if not all should be covered under preventive care for "well child" if you are still in high school under your parents' plan even if you are 18. I was happy and surprised last year that the few that DD needed to get prior to I-Day were 100% covered, even a couple of "odd" ones. The only requirement was that she have a wellness exam, then immunizations that day and subsequent within 12 months were covered. Even the varicella-zoster virus (chicken pox) titer and TB tine were covered.
As runnergirl said, you don't want to put any off for I-Day. A sore arm or arms would not be good. You will be carrying a LOT of stuff around the first day and push-ups/pull-ups would not be much fun either.
I don't know which immunizations are required this year, but note that some may be optional as well; others are mandatory before you can report.
Ours had to get a few shots and we also did it ahead of time and it was covered by insurance. I do recommend they say it is necessary for college and not the military because military medical requirements aren't covered under most policies.
^+1 Agree with Daretodream, and this is not necessarily deceitful since most, if not all, appointees have applied to and/or been accepted to a college. If you check the colleges' immunization requirements, they are similar. So, you will probably need immunizations either way. I know DD's plan B college required most of the same ones with similar optional, but recommended ones.
Also some shots are recommended to be given 30 days after others. For example, pneumonia shot 30 days after flu shot. DW recommends MenB (don't tell the kids but hands down the most painful shot) in addition to MenA as cadets live in a dormitory setting. Some might remember the MenB outbreak at Princeton a few years ago where 1 student died. While the shots for the AFA take priority, here are the recommendations from the CDC we all should consider:
Of course, immunization is a loaded word these days, well, for some decades really. My kids didn't seem to mind about getting the shots over an 8-10 week period. Our pediatrician knew our objections (and shared them), and since three of her kids went to USAFA, she understood the timeline and protocols.
While at USAFA, I believe there was a required "flu shot" as well, which IIRC was later modified to the flu nose spray.