Gaëlle Seguillon has a great collection of reference images on his Gumroad that you can purchase or grab for free.
Here’s some great packs being sold for only 2-3$:
All of the images are high-quality jpegs from uncompressed RAW at native camera resolution (6000×4000).
There are much more high-quality references on the artist’s Gumroad page. Some of them can be downloaded for free, however, if you want to support Gaëlle, you can always offer a fair price for the pack that you really liked.