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. Recently the site has started uploading packs again.
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 Official 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.
https://itgdb.s1sh.xyz/ is a more detailed version of the itgpacks.com packs list, which provides information about the songs & charts within each pack. You can do filtered searches of the database as well. The packs are not directly linked from the website, so they will need to be downloaded elsewhere.
nnty.fun/downloads/game/stepmania/stepmania_packs/ is a giant database of packs that are alphabetized in a one page list. It is recommended to use your browsers search function to locate specific packs. Like the SMO database, a ton of the content here is not for ITG, so if you're not careful you may end up downloading keyboard packs.