Where is a good source to find aftermarket putter headcovers? I have a redwood anser and would like something different in terms of the headcover.

Here - For Sale section.
Golf Stores.
eBay.
Search The Internet.

My choices would be....

GolfWRX cover...some neat designs available through this site.
Come Como cover...very interesting and busy covers...should be able to find them on the internet somewhere.
make your own.....get a AM&E blank cover, buy some patches from ebay, secure them (glue, ironing, both, etc.) somehow....you now have a one of a kind headcover. I have a couple of these.....easy way to get just the cover you want.
Ping WRX cover....I was able to order one through a sponsor that deals with Ping.
Plus, I really like the new Bettinardi volcano cover but I'm not sure you want to mix companies like that.