I think using OneNote to locate the customer record may be a little
difficult, however if you definitely want to use OneNote you could set up a
database with access, that links to the indiviual customer file. This would
cause OneNote to open with the selected customer.
Within the folder My Notebook, that is located within My Documents are a
series of files with the extension .one Each of these file corresponds to a
section, so by giving each customer a section you can use the method I
suggested.
You could also build in a feature to create a new sections when you add a
record to your Customer database.
However I would be a little concerned about how well OneNote would handle
that many files.
Hope it helps