Legal up to date short data is available but costs big $$$ to access, although your broker may share the info with you if you call. Daily changes can be found via links from this site:
Click on the links AH and download the various .txt or .csv files from ach MM to see the short trades they each did during the day - culling this data automatically is how the Shortanalytics website creates it's daily charts.
However, even for the pricey daily total short data I mentioned above, that's still an incomplete view of true short numbers - it doesn't count phantom/naked shares trading in "dark pools" - those shares can account for a huge % of unseen shorting. For ARNA shares, there are strong indications that many shorts are phantom/naked with multiple owners - but don't expect to see a short squeeze on approval from those shorts - since they actually don't exist there's no reason to cover them.
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