Thermal pads are typically sold in sheets which can be cut down to size. If you are looking to replace the thermal pads between the VRM heatsink on your X299 motherboard, you will want to ensure the thermal pads are the following thickness:
X299 Dark: 1.5 +/-0.2mm
X299 FTW w/FAN: 1.0 +/-0.1mm
X299 FTW w/o FAN: 1.5 +/-0.15mm
X299 Micro2: 0.5 +/-0.1mm
X299 Micro: 1.5 +/-0.15mm
If you have any further questions, feel free to contact our technical support team by calling 1-888-881-3842 or e-mail firstname.lastname@example.org.