110 lb is plenty thick enough. If you've already got 110 lb, use it.
But 67 lb is what's usually done. You can buy it in white or a variety of colors at Staples, Office Depot, Office Max etc.
If you can't find card stock you can use manilla file folders, 8 1/2" x 11". Use a paper cutter to trim the front of the folder down to 1 inch, and trim the back of the folder to 8 5/8 inches. Then trim the sides to a length of 11 1/8 inches. Use a 3 hole puncher to punch holes in the one inch strip for the brads to go through.
Do this twice to make two covers, on for the back, and one for the front. The one inch strip covers up the brads, just like those spiffy agency covers.