Here is a list of sites with further information and a good way to discover new pieces, I have spent many hours on many of these sites in the creation of this list; however, they still include fresh perspectives and pieces I overlooked or haven't put on the full list yet.
Hathi Trust - ~3.31k pieces, unorganized and many are not exactly relevant for string orchestra, but unique mass of pieces mostly with available scores
Estonian Music Information Center - ~500-600 pieces, pretty organized but scores, recording, and some basic info can be hard to come by
Mainly 4 Strings - ~200-300 pieces to guess, mostly older/more traditional form string music
Philomel Project - ~400-500 chamber (players<7) by female-composers, mixed in with other music but very informative and helpful, good composer list
Classical.net - Composers list
Institute for Composer Diversity - 1000+ composer database, true gem for finding new composers.
American Orchestras - List of databases of repertoire by underrepresented composers
Dakon/Williams List - Good string orchestra list, especially for Texas educators, very similar to my lists
TMEA Prescribed List - ~1500, Great list but mildly inconvenient and much bigger focus on older music
CMA Composer Equity Project - Large composers list of ALAANA, Women, and Gender Non-Conforming Composers
Gen Gaku - -2000 pieces, mostly classical and older pieces, not much info on a lot but good list
Luck's Music Library - ~100-200 pieces, trying to sell you their music, but their music is great.
Yale Library Music Diversity - List of databases for diverse repertoire
Devlyn Case Repertoire Lists - Collection of multiple repertoire lists, good resource
MMEA List - ~900 works, Maryland music educators, great list and organized well, similar to Texas with a focus on more known and older works, some interesting grading decisions but nevertheless a useful resource.
Wise Music Classical - ~1000 works, good but often no recordings and for profit.
Zinfonia - Search "string orchestra": ~500 works usually newer and for rent
Sounz - ~3700 works that mostly look pretty unfamiliar, looks like an incredible resource.