For HO, the list is long, wide, and finicky. Time was that Details West, Canon & Co., and Detail Associates were the big makers in this space. Now, detail options are significantly wider, depending on the details. You now have Cal Scale, MAC Rail (through Midwest Model Railroads), some remaining but out of production BLMA parts, A Line, Gold Medal Models, Jaeger. Walthers has detail kits for most of its modern diesels, and most manufacturers sell parts direct.
There's even a Diesel Detailer forum -
https://dieseldetailer.proboards.com/forum