You will have buy a replacement chinstrap/fit-band assembly.

It can get tricky because which one to get depends on if it is a "NEW" or "OLD" helmet and Ops-Core has started calling everything "High Cut" whereas in the past they used "High Cut" and "Super High Cut" because they are discontinuing their legacy "High Cut" helmets.

The legacy FAST helmet (high-cut without the Lux Liner, not super high-cut with the Lux Liner) replacement assembly P/N is Gentex P/N family 60-95-XXX. The description is: Replacement: FAST, Carbon, Bump, Retention & Front EPP Pad, Head-Loc 4-Point Chinstrap - OCC-Dial, Right-Eye Dominant, includes 15 mm Hardware Kit. The "XXX" in the P/N is numerical and varies depending on the color, size, and RED (right eye dominant) or LED (left eye dominant).

The new FAST helmet (Super-High-Cut with Lux Liner) replacement assembly P/N is Gentex P/N family H10758-XXY (X= numerical, Y=alphabetical). The description is: Suspension/Retention: FAST, no hardware included, Vented Lux Liner Padding, Head-Loc 4-Point Chinstrap - OCC-Dial, Low Profile,  Universal Fit-band, Right-Eye Dominant. Similar to above, the "XXY" varies depending on the color, size, and RED (right eye dominant) or LED (left eye dominant).

And to make it more confusing, there is also an H-Nape P/N variant

We have the replacement assembly for the NEW FAST BUMP (also compatible with all NEW FAST helmets) on our website that you can order. I'm unsure if I can post the link so the "easy button" to find is just goggle this: TNVC OPS-CORE FAST REPLACEMENT OCC DIAL UNIVERSAL FIT-BAND (LEFT EYE & RIGHT EYE DOMINANT)

If you need the legacy version then search for that P/N family above - google: GENTEX 60-95- and that should locate something. If you can't find it elsewhere and/or want to verify with me, just shoot me PM or give me a buzz next week. I need to know your helmet size and color. If you're not sure about the size, on the inside on the shell it will be stamped and there may also be a Gentex sticker with P/N and description.