I don't believe that any royalty-paying business could fail to know how many units they have sold, and stay in business. However, there's no reason to expect that this information will be freely made public. It's fairly sensitive business-critical data. Even less reason to expect to find some cozy online database that tracks it all for more than a few willing publishers or titles.
You might check Publishers Weekly to see if there are any mentions of sales reports from big publishers.
You could probably ask a publisher about a specific title, but I think you'd need some kind of reason for asking. If you were the author, by all means, they should have info for you. Now, if I were to get nosy and ask about books I have no legal interest in, I would expect to be blocked by privacy concerns, or at least fail to make a case for needing the numbers.
I don't know that you can expect a tally of records from decades ago -- very few businesses can keep their whole history at their fingertips. Old records take a lot of time to digitize, and if it will cost a half million dollars to scan tens of thousands of documents that you don't expect anyone will ever ask about, why do it? Surely, among day-to-day operations in the 1990s, a royalty statement for a book that's been out of print for 20 years is not a high demand item.
Although you'd never guess it from looking at the web, only a small percent of the paperwork from before the Computer Age has been digitized.