The source is a Shevet Halevi (Rav Vosner), who says that Pants are inherently a Beged Ish. L’cheorah tights are included in this as well, as the form of the “pants” does not make a difference (as per the Shevet Halevi (chelek 6, siman 118).
P.S. Rav Vosner says in the same Siman that Snowpants that are specificly worn to protect from snow are allowed to be worn (are not Assur) by a woman witout a Skirt. Ayin Shom.
Are you looking for a source that “shok” includes up to the knee?