Ah! We all wish there was an
Oakeshott-type organization daggers, but, alas, there isn't.
You can get a blade that is both stylistically similar and historically plausible - it just takes some research to find something.
Logan Thompson's book Daggers and Bayonets is a pretty good reference.
Again, narrow your interest and sometimes then the answer becomes obvious.
Is your sword just something for a collection? Then just get a dagger from the same time period that looks good to you.
Are you building a persona who would use this sword? Then research that persona in detail. Where did that person live? What class of citizen was that? What did that person do?
I'd start with Google and just start searching, both websites and images. Search on "15th Century dagger" and see what you get. Look through the recommended book list here at myArmoury and buy a few books that cover your time period of interest. Read a little about the history of that era.
Once you start looking, you'll find an explosion of other things to look at. I keep a Word document to help me keep track of what links I've seen, what links I want to see, and what links I want to keep for future reference.
EDIT: If you want to make an educated choice, you have understand how and why daggers developed the way they did; you have understand the armor in use at the time and how daggers were meant to defeat that armor; and you have to understand who would use what type of dagger.