Songs are added to StepMania in the form of "simfiles" which contain one or more "charts" and an actual audio file. These files are usually grouped in "packs" (which some themes refer to as "groups"). You can make your own charts or download ones created by others in the community. This page outlines some resources to find packs and simfiles.
One thing to keep in mind before downloading stuff is that not all StepMania players are playing ITG (that is, 4-panel play on a dance pad). You can use StepMania for other game modes or for use on a keyboard, in which case those charts are not designed to be played on a 4-panel pad. Some resources specifically cater to ITG to make this a little bit easier.
itgpacks.com redirects to a Google Sheet which is a curated index of ITG packs. All of the packs included here are for 4-panel pad play, so you don't have to worry about accidentally downloading a keyboard-player pack. This resource is generally the most up-to-date and its method of categorizing packs and sorting the newest ones is very helpful.
stepmaniaonline.net provides a search tool which is very handy. You can search the contents of a huge database of packs for specific artists or songs. However, a ton of content here is not for ITG, so if you're not careful you may end up downloading some keyboard charts.
The GrooveStats ranked pack list is a spreadsheet that contains downloads to all packs ranked on Groovestats maintained by StarlightLumi.
Zenius-I-Vanisher is a arcade forum based around rhythm games. You can find Dance Dance Revolution Packs as well as simfiles made from the community under Games>DDR & DS>Simfiles.
The Etterna song pack repo is another good resource to find links for packs that have broken links or slow downloads. Be aware that most of the packs listed here are meant for keyboard, as Etterna is only for keyboard play.