LogInNoEmotion HDRs might be the best place to look for free skyboxes.
NoEmotion HDRs might be the best place to look for HDRs. It is basically a treasury of free HDRs, which you can use for both personal and commercial work. The developers spent 350 hours to create 150 HDRs. Images were taken over a period of roughly 10 months.
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The collection includes day, evening and night scenarios. Most of the pictures are said to be taken around Prague and Czech Republic. A few mountain shots are from Swiss Alps. You can use these HDRs with any render engines.
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Each image is 300MB and there is a bandwith limit of roughly 1TB per day, so please don’t download everything at once. You can find the breakdown of the creation process and other details on the collection here.