You need DB15 on both ends/sides, correct?
I have a ~1 meter cable with a DB15 (with thumbscrew fasteners) on each end. Used it with a Mac mini video out to DB15 adaptor which won't be of use on the Quadra. All my other cables/connectors have some kind of (newer) Apple video out plugs.
More infor: The DB15 connectors may be called "SUB", they have 3 rows of 5 pins each. A "normal" VGA port can have 2 rows; 8 on the wide part of the metal "sleeve" and 7 on the narrow part. Not sure what the port on the Quadra looks like.
If it looks like the one at
this "Mac Ports, past and present" link, my cable should work.
Otherwise, you may need one that look more like image on the right-hand side at
this site.