If you really want this, check here
. Minimum order is $384 for one, but you can get a discount of only $18 per--if you buy 25 of them at $451. Such a deal.
You are probably better off drilling out the back of your computer case and mounting your BNC that way. The only other thing I can think of, and this is if you really
want to do this in a PCI bracket, is to just drill a PCI bracket yourself. I reckon this could be done, but most brackets have relatively soft metal, this coupled with the small size of the thing means you risk mangling the bracket at least as much as making a neat hole. Just find a good wide space in the back of your case and start drilling, get your BNC plugs, mount, then wire to the plugs.
Now that I think about it, if you could find a PCI card of some kind, maybe a cheap, junk, nonworking card that just has the number of holes that you require, you could just cannibalize it by taking the bracket off and mounting your own BNC wiring on there. I'd probably drill and wire to the case just for expediency.