Where can I find databases of correspondence games? I prefer freely available ones, but commercial products can also be suggested.
Congrats on your request for finding correspondence games. These are pure gems as today you can expect these games to be examples of almost perfect chess (yes, all correspondence players let the computer do 99% of the work). You can also get databases of computer games at longer time controls to get the same 'perfect games'.
The go to database for correspondence is probably the disk that ChessBase sells and they just updated it. http://shop.chessbase.com/en/products/corr_2015?Ref=RF70-5OH0EFN840
A while ago, Tim Harding put out a disk as well, and it may not have current games, but you never know until you check. UltraCorr CD-ROM (Chess Mail Ltd., 2006). Followed by Ultracorr2 (2008), UltraCorr3 (2009), and UltraCorr3a (2010). No longer available. I am not sure about the no longer available part, but google Tim Harding and ask him!
You can also download games from the ICCF (the granddaddy of all "correspondence chess") which as you may know runs the REAL World Championship. ( www dot iccf dot com) and have a look around.
Sorry I am only able to post 2 links per answer at this time.
Lastly there are probably tens of thousands of places to download games if you google it. Here is a really good one! http://chessgameslinks.lars-balzer.info/#correspondence-chess_worldwide Now if you drop off the part after .info i.e. chessgameslinks dot lars-balzer dot info/ you will have enough to keep you busy even in the afterlife!
The Internet Email Chess Group is now hosted by the Lechener SchachServer. They have collected over 400,000 CC games, which you can download for free in ChessBase or PGN format, after registering for a free account.
I bought Correspondence Database 2013 from ChessBase, and it has over 1 MM games. You can often get a slightly-out-of-date CD of CC games on ebay at a steep discount. Mine came as part of a product package containing ChessBase 11, which I got at a discount once ChessBase 12 was released.