Both of my used Model 3's did not come with license plate screws. If you're in the same boat, the size you need is 5mm 0.8 x 6mm if you're mounting it without a frame. I use a clear cover and frame, which makes it a bit thicker. Thus the screws I used are 5mm 0.8 x 10mm. This was a perfect fit for my Model 3. I believe the Model Y is the same. I don't know about the Model X or Model S, but I'm pretty sure they use a different size. The Tesla provided screws are black and these are silver. I don't really care about the color.
I bought it from Lowes and it was about $2.50. Amazon and Ebay sell them as well, just look for the right size.
I bought it from Lowes and it was about $2.50. Amazon and Ebay sell them as well, just look for the right size.