Reddit reviews Selected Duets for Saxophone, Vol. 1: Easy to Medium (Rubank Educational Library, No. 194)

It's obviously going to all come down to personal preferences, but if you're looking for more modern, recognizable stuff (particularly if you're looking at busking), you might take a look at Chromatik -- they've recently rolled out a subscription-based plan, so I'm not sure how "free" it is at this point, I think if you're willing to sit through the advertisements, you can still get at all the sheet music.

I've generally had good luck using Google Image search to lead me to places to find the sheet music I'm looking for, just search for <song name> tenor sheet music, and in general, you're pointed to either a free or low-cost option for finding that song.

I haven't made a serious search into finding duet music, but I have been using the Rubank Saxophone Duets book with my daughter, who plays clarinet, while I play my tenor. I also play in the Horn Players Jazz Jam at my local Sam Ash store on Sundays, that seems to vary by location, but at ours, we use primarily the Hal Leonard or Jamey Aebersold play along books & backing tracks.