A day out on the airsoft field, indoor or outdoor, is some of the best FoF training that sane amounts of money can buy.
Sims are theoretically better, but unattainable to most people.
$60 at American Paintball Coliseum on Saturday morning with a rental gun and gear will have you absolutely shitted by lunchtime and give the reps to internalize some theoretical training concepts you've learned. You'll be playing against kids, but they're wily, unpredictable, small, and have been raised on a steady diet of Modern Warfare their entire lives. It's probably more structured than most real-life gunfights on the street.
If you go with some competent guys, I don't see the experience as appreciably different from UTM sims. The fear inoculation is real, if you let it be, and identical to times in my life that I've legitimately been in fear for my life.
$60 in any flavor of sims gets you maybe two magazines of ammo. This assuming you have the gun, the gear, the venue, and enough dudes similarly motivated.
Maybe there should be a co-ar day at airsoft.
If you want to get into the game on a serious basis, figure $300 for something like a basic quality KWA rifle, and $150 for support gear, including your first bottle of BBs and a LiPo battery setup.