Its publisher's website:
http://chessbase-shop.com/en/categories/49
buy it through chess.com's site (or whichever has the better deal):
http://www.wholesalechess.com/store/chess_software/fritz13?sort=featured
Note the prices are about 40% cheaper at chess.com's site.
sftac
ps. It's been around for a couple of years (so, expect not to pay full retail, eg. it's even cheaper on eBay if you look closely). I've noticed it seems to lack a good 'manual', so it sounds like a Lot of work to figure it out and use it. Here's a review of Fritz 13:
http://tacticstime.com/chessbase/fritz-13-review/
Does anybody know?