For the source text click/tap here: Shekalim 3
To download, click/tap here: PDF
The Mishnah opens with the statement that on the first day of the month of Adar - they: the Temple priests and other Jewish authorities - proclaimed that the payment of the half shekel will soon be due in just one month - the first day of the month of Nisan. On that first day of Adar, the Jerusalem Talmud states that heralds went forth from Jerusalem proclaiming that the tax would be due in a month.
Now, why were there money changers in the provinces, and why were there money changers in the Temple? Because the tax was 1/2 shekel, and when people were ready to bring their shekels from their homes to Jerusalem, they needed to make change - you gave the money changer a shekel and he gave you two 1/2 shekels and you could then take your 1/2 shekel to Jerusalem to pay your tax.
This leads us to the commonly helld belief based on NT sources regarding the corruption of the money-lenders and the Parable of the Pound.