I am looking for an old stock price of a company that subsequenty has merged with another company. I am looking for the stock price of Union Electric (UEP) on Oct. 12, 1992.
Go to a large reference library and ask to see the Wall Street Journal for October 13 1992.
I've had luck finding old stock information in the Google scanned newspaper archives. Unfortunately there does not appear to be a way to search exactly by date, but a little browsing /experimenting should get what you want. For instance, here's a source which shows the price to be 36 3/4 (as far as I can read anyway) on that date.
A quick search showed me that UEP merged into Ameren on Dec 31, 1997, and Ameren still exists today. So I took a look at Ameren's Investor Relations website. Unfortunately, they don't provide historical stock prices prior to Ameren forming, so starting with 1998. However, I've had good luck in the past emailing a company's investor relations contact and asking for data like this that isn't on the website. It's reasonably likely they'll have internal records they could look it up within.