The City of Boulder has recently published a complete database of all Licensed Rental Properties in the form of a CSV spreadsheet file.
You can find the official source here: https://bouldercolorado.gov/open-data/rental-housing-property-list/
One great piece of information contained in this file is that rental property entries include a neighborhood field – so you can easily get a list of all the rental properties in a specific neighborhood (example: Martin Acres currently has 482 licensed rental properties in single-family structures).
We’ve converted that file into a Microsoft Excel spreadsheet, making it easier to work with. The data set is quite huge – click on the icon at the bottom right corner of the window below to view it in a full-size browser window, or download it to your own computer.