There are no kidding NVIS compliant filters that you can buy but they're extremely expensive. We're mostly using toughbooks now, so when not in use they just get shut to keep the screen off as the filters made for the toughbooks prevent the touchscreen from working (hard plastic offset from screen), so not even an option with an iPad. I've seen some made from a sort of soft plastic filter that used a leather surround and elastic straps to hold the filter on (get a local shop to sew em up or do it yourself, but you have to buy the filter) which worked great, but I don't know where the filter material came from. Photography gels are really cheap ($5 for a 20x24" sheet, Rosco is one of the big makers) and thin enough that they shouldn't screw up the touch screen, and you could just tape or velcro them on but you'd have to experiment with colors to figure out if that would be "good enough" rather than the legit NVIS compliant filters.
On that note, I'm going to order some gels and do some experimentation of my own. EDIT: Going to give this a try: Roscolux #389 Chroma Green