Where do we find that King Shavur would take specifically from the rich who profit from charging interest?

Ben Ish Chai: He would only take at times of hunger. At such times, merchants are also not profiting. The only rich who are profiting then are those that charge on interest. This drew the attention of the king and he took from them and used it to sustain others.

What exactly did Shavur do with the money?

Maharsha: Rashi in Mishlei brings from the Midrash Tanchuma that a 'Chonen Dalim' refers to building bridges for communal benefit. This is why the Gemara here refers to Shavur Malka, who was a Persian king, as the Gemara in Avodah Zarah 2b describes that in the future the Persians will ask to be rewarded for building bridges.