How can I close off crawl space vents for the winter?
Your louver (damper; shutter) doesn't  work so well anymore. Replace the whole thing with one made out of metal that has rubber seals where the blades contact: (solerpalau-usa.com)



A working louver should not leak (much) air or need an extra cover, however you may want to shove insulation or foam board in it, for over the winter. A custom cut-to-fit foam board taped-to it on the inside, may be a cheap temporary solution. 

FYI, a vent cover with movable lovers is also a register. One without movement is a return grill, or simply a grill.



If your grills are metallic and relatively flat, you can use a magnetic vent cover: (always-free-shipping.com)