The best cork I have ever seen came from Angler's Workshop, but you pay an arm and a leg for good cork.
If you can't find really good cork, and you may not be able to, as it's getting harder and harder to do, you may have to settle for some fill. The best fill I have found, and I have tried a lot, is Elmer's indoor/outdoor carpenter's wood filler. It looks like this, but is the indoor outdoor version. Make sure it's golden oak. It matches cork better than cork.
I should add, the last cork I got from them was nice, but not as good as the previous batch. It pays to ask how good it is before buying it.