Entirely unknown types do turn up, but not very often. Below are a few examples.
A few years ago I saw a new type for Helena with the reverse of HELENA AVGVSTA. A second example popped up last year.
The second picture below is a Constantine I with an unrecorded reverse of FORTVNA REDVX AVG N that turned up in 2011.
Sometimes even common types turn up that are really different, like the Licinius II 3-D campgates from Rome with VIRTVS reverse.
I don't know that any previously unknown types are actually somewhat available though. These rare types will always command high prices in auctions...your best bet is to hope to find something that nobody else notices, maybe in a large group, possibly even photographed poorly. It's like winning the lottery though, the odds are against you, but it could happen.