Center for Public Integrity - https://www.publicintegrity.org/
A project to follow the money and spending of PACs and political donors to candidates and ads.

Federal Election Commission Campaign Finance Data - https://www.fec.gov/data/
Current site for the FEC Portal https://classic.fec.gov/pindex.shtml which is still being maintained but I am not certain for how much longer.

OpenSecrets.org -  https://www.opensecrets.org/
From the Center for Responsive Politics who track money in U.S. politics then use this site to report how the money affects policy and citizens' lives.  It is fairly comprehensive in how money and politics are intertwined.
Iowa Campaign Contributions - https://data.iowa.gov/stories/s/7h4k-xejg
This report provides information on monetary contributions and in-kind donations made by organizations and individuals to state-wide, legislative or local candidate committees, state PACs, county central committees, state parties, and state and local ballot issue committees in Iowa.

Candidate Instructions on Contributions, Iowa Ethics and Campaign Disclosure Board - https://ethics.iowa.gov/campaigns/candidates
This site provides the rules for candidates and volunteers who perform campaign services.

Campaign finance requirements in Iowa, Ballotpedia - https://ballotpedia.org/Campaign_finance_requirements_in_Iowa
A non-profit site with links to federal and state campaign finance rules.