You can buy IR filter material from optical component suppliers such as Thorlabs and Edmund Optics; both sell longpass/unmounted filter glass that can be cut and mounted in place of the camera’s original filter. If you’d rather avoid cutting glass yourself, LifePixel sells camera-specific replacement filters for Nikon bodies and offers different conversion types such as IR-only, UV-only, or full spectrum. Their filters are pre-sized, which is usually easier and more consistent than cutting raw glass yourself.

One responder also mentioned MaxMax as a source of camera filters, but reported inconsistent quality on a front-mounted filter, so that recommendation is less certain.

For a DIY internal conversion, a pre-cut replacement filter is usually the simplest route. Also be aware that after conversion, autofocus may need recalibration.