You might want to check your local restaurant supply store as well. If it doesn't have to be a keg top, you can often find open stock lids that will fit well, or you can roll your own from a pizza pan, a handle, and a few bolts.
My lid came from a 48 qt. stainless steel canning pot, and it fits perfectly. It definitely helps heat strike and sparge water faster, as well as keeps junk out of the wort while it's chilling.