Metacortex

Those are Schwinn Approved "flexon" (vinyl) handbrake lever covers, made for Schwinn by Hunt-Wilde and also sold under the Hunt-Wilde name (and a few other names as well). They were introduced mid-1968 in colors matching the colored handlebar tape and grips Schwinn installed on its bikes, which also matched most of the Schwinn paint colors at the time. Various colors were added and subtracted over the years until they were finally discontinued in '82. Fanatics of Schwinn '60s and '70s models (and the paint matching colored tape/grips) often consider these an absolutely essential accessory to install.

While certain colors are very hard to find, the more common colors are almost always available on eBay: http://www.ebay.com/sch/i.html?_nkw=schwinn+lever+cover