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.