I found them at Lowe's. At least at our store, on the aisle with the screws, bolts, nuts, etc, there are drawers with odd specialty items like that. The screw covers they sold me are one piece, with the base and the cap connected by a little bit of plastic. Easy enough to cut off the cap so that you do not have to disassemble the window latch mechanism.
I cannot remember which size worked, but they are cheap enough that I bought a couple of different sizes to make sure that I had the correct one when I got back. I might have some left overs, so let me know if you need me to dig through my spare parts bin to get you more info.