Thanks for your question and for joining here on the community.
When you purchase (at least from us) a TV mount, the mounting screws are included in the box.
They are typically at or around 1/4" diameter and no more than 3" in length lag screws.
The key for any installation is making sure the mount is fastened DIRECTLY into the wall studs. Using a stud finder is extremely important, as it will show you the on-center location...the best place to install the mount.
After finding the on-center portion of the stud, using a smaller diameter than the screw hole pilot drill bit helps guide the screw into the studs for final installation.
If you already have a TV mount without the lag or install screws, your local Home Depot will carry a plethora of screws to best suit your needs. Just make the head of the screw isn't smaller than the final hole of where the mounting holes will be once fastened into the studs firmly.
Let us know if you have any further questions,