As some pages of my site have gotten n/a of PR from Google, I need to see the history of PR's changing for those and other pages to estimate the situation and decide what to do. Where can I get the past PR's of exact site/page?
The reason that many pages on your site have no PageRank is that Google hasn't released data for any pages created since December 2013. Google occasionally creates a snapshot of all the PageRank on the internet. That snapshot is what you see in the Google Toolbar and other PageRank checkers.
Google doesn't offer a history of PageRank, just the value as of the last data refresh. The only way to have gotten the historical values would have been to record them yourself.
Because the toolbar PageRank data is so stale, you shouldn't worry about n/a PageRank. There are other metrics that indicate that Google is happy with your site and your pages. I stopped paying much attention to PageRank and I use indicators like: