As many have noted, the trainee outfit is built around a light-blue, long-sleeved henley t-shirt, with khaki cargo pants (optionally bloused/tucked), combat boots, red training gun in holster, and a badge.
There were plenty of reference photos:
|Up until now, I didn't realize the male and female cargo pants were different shades.|
(Incidentally, if you Google search on "light blue waffle henley shirt," there's an important two-word phrase in there that may pull up some verrrrrrry different search results than you might be expecting.)
I had everything else (after spray painting red the toy gun from my 2010 sheriff costume), and made the badge in photoshop:
|The fonts are a bit off, but honestly, who cares?|
As to the finished product, over three wearings in three days, exactly two people knew what the costume was... and one of them was an actual FBI agent. Also, the holster is a pain to drive in.
So, I've used up my fallback costume, which means that I have a year to come up with something for 2017.