When the firstborn Egyptians died during Makkas Bechoros, the Plague of the Firstborn, Yahweh spared the Hebrew firstborn from a similar fate. At that time, the firstborn were consecrated, and chosen by Yahweh to be Kohanim – to perform the sacred service in the Bais Hamikdosh.
But the firstborn lost their status when they sinned with the cheit ha’egel- the Sin of the Golden Calf. The Kehuna was given to the children of Aharon. But the firstborn males had been chosen by Yahweh. They could not simply behave as if they had never been chosen. That is why they must be redeemed with a Pidyon Haben.
The answer to question one was not too difficult. Google "Pidyon Haben Coins" and various links will pop up or "Shekel" and you can go to websites where people have begun minting coins for use once the Temple is rebuilt. I personally liked http://www.pidyonhaben.org/ because it gives a detailed summary as to the modern day observance of the mitzvah which gave me a little background, although I simply will follow the Torah guidelines for me personally. (Images courtesy of http://www.pidyonhaben.org/ as well)
Bible References: Exodus 13:13, Exodus 22:29, Exodus 34:20, Numbers 8:18, Numbers 18:15)
Oh, and just as an additional reference on the ceremony from a Messianic standpoint, check out this pdf file: