Look at the old (1980s onwards) crystallography papers from Don Wiley and John Skehel on HA - they purified biochemically homogeneous preparations of trimer. I don't remember the method beyond them using protease digestion to release the HA from purified virus, but somewhere in that literature you'll find a protocol.

Or for recombinant HA, take a look at the papers from Ian Jones at the University of Reading:

https://www.reading.ac.uk/biologicalsciences/about/staff/i-m-jones.aspx

Good luck!

Paul

 Thank you, Professor :)