Roman nails are
very common finds on
Roman sites.
However, they are often seen as almost valueless. They rarely appeared in
museum collections because they were considered mundane and boring. That is changing now with better displays so you often see some nails in
Roman construction or
Roman metalwork sections.
Likewise they are rarely offered to collectors - not because they are
rare but because they are usually weeded out of lots as junk. That said the images Alex gives shows typical
Roman nails and you can find all these for sale if you look. They are just hard to come by.
In terms of crucifixion there is only one piece of archaeological evidence. A skeleton from the Givat ha-Mivtar district of Jerusalem was found with the remains of an iron nail through the right heel bone. You can find many images of this bone with the 7", square cross-section, roughly square-headed nail on the internet.
I have a
Roman nail about this size in my
collection. I also used to have a whole bunch of railway spikes when I was younger. I used to gather them from along an abandoned railway bed near where I lived. Though railway spikes are about the same length as the Givat ha-Mitvar nail the railway spikes are much more massive - much thicker and at least 4 or 5 times the
weight. So don't be tempted to take a railway spike in as an analogy. If you can't find a
Roman nail then a 6" or 8" spike from a garden centre would be a better analogy for display.
Shawn