The concept defending points is not exactly true, players are not defending points, as you well said, it's a 52 weeks rolling system and points from the week 53 are dropped off while points from the week 1 are added.
They say defending because that's the number of points they are dropping off and they are related to their perfomance in the tournament, it's not a very good way to call it in my opinion. the meaning is that the player should at least get the same result in order not to lose points in the rankings, which is not true since many factors influence it.
About where to find info, well, I personally have kept old rankings from the end of 1998, in the column points added you have how many points each player has earned with each rankings release. I have the last 52 weeks in an spreadsheet. I don't know of any internet site that offers historical information on the rankings