What are you looking for exactly and for what bike? Except for a few things like fork bottoms/calipers, decals wheels, frame color and harness/regulator style (single/three phase) the bikes are basically all identical.
In other words the headlight bucket off a 2002 998 will fit a 1995 916. the biposto subframe from a 1997 748 will fit a 2001 996 and so on. The basic structure and hardware remained pretty much unchanged for the entire production run of the 916 era bikes.
Honestly ebay is probably your best source for cheap stuff like relays, harness bits, etc. Hell I just bought a complete set of clean 748R heads (minus cams) for $360!
If you can tell us what you're looking for and for what bike, I'm sure there's also plenty of folks here including myself who may have what you need kicking around in the garage.
The typical dealers will sell you any part that is in the catalog, they just don't have online parts manuals that go back that far. Get a parts manual for your bike, enter the part number you need, and most of the online stores will come back with info on the part, quite often the new number that replaced the old one.
You have to be careful though, some things get lost in the shuffle. I've seen one or two updated parts for my 851 that aren't even close to right.