You can also use brass or stainless steel. Take one of the screws to the local big box hardware (Lowe's, Home Depot) and in the hardware area they have a device you can screw into to determine the size and thread count. Then you just need to find the right length on the shelf. It's probably about $2 for a pair of them.
You can get them on Amazon. Max size is M6x16mm. Any longer and they will interfere with the back window. Like Rich mentioned, I use stainless cap screws and vinyl caps.