Well, some google-fu may be in order.
I imagine there are very few people looking for that particular part (25 years after the fact), and and fewer still posting individual spare parts online for it.
You are talking late 80's, probably 87 or 88 im guessing. Search for date codes and get an idea of when it was sold. There were few manufacturers and models of PC's back then. The processor is an i386, so Apple is out. That is a start. Then just narrow it down from there with specs. Do you have/know the power supply? that could narrow it by region. Add up the RAM, and max RAM, HDD size, etc. That should give you a short list of computer models sold at the time. A quick check on the specs of those will probably lead to exactly one candidate. Then, you should have a much easier time searching for that model computer that you can salvage parts from.