It would help if you told us the size of your tank and brand.
Look for "Versa-Top" glass lids made by Aqueon.
I believe Petco sells them, among other online sites.
The Aqueon website list all the dimensions of the Versa-Top lids, so you can make sure you get the tight one for your tank.
I've bought one before, and would do so again if I needed another lid.