Chris,
Ah, if only it were as easy as simply specifying either 1U, 2U, 3U, or so on for rack ears and placing an order.
But each model series seems to have different ear types.
I've used Ebay, of course, to get rack ears. But first I try to find an Ebay pic of the router or switch with the ears already mounted. Just to confirm the hole pattern.
Here's a good selection.
http://www.ebay.com/sch/i.html?_nkw=cisco+rack+ears
Add your device model or series number to the search string to narrow it down.
There are also specialty stores on Ebay or online that have Cisco accessories with the appropriate selection tools.
I haven't found a nice part number listing that gives a table for all the routers and switches ever produced. THAT would be handy.
Good luck!
__Will in San Jose