Where do airshow pilots practice?
Aerobatic pilots typically use "aerobatic practice areas" when they practice.  Many of the Part 91 rules which would need to be broken in order to practice aerobatics are waived within these aerobatic practice areas (such as the 1,500 ft. "floor" for normal aerobatics in 91.303(e)).

The FAA has to approve them, and their guidance includes a good description:


  4. AEROBATIC PRACTICE AREAS. Aerobatic competition pilots, airshow pilots, and others who wish to practice aerobatic maneuvers not
  necessary for normal flight and below an altitude of 1,500 feet above
  ground level (AGL) must use a waivered aerobatic practice area.  These
  areas are not to be considered airshow sites.  The aviation community
  uses these practice areas to establish and maintain proficiency as
  well as enhance competitive skills in all the recognized aerobatic
  maneuvers. They are established by the waiver applicant in conjunction
  with the local FSDO and may have dimensions of several miles in
  various directions or be as small as a contest box; i.e., a cubic box
  with a dimension of 3,300 feet on all sides. Inspectors should be
  receptive to the establishment of these areas, consistent with safety
  and the efficient use of the NAS.  It is imperative that the safety of
  all nonparticipating aircraft be considered when issuing a Certificate
  of Waiver or Authorization for an aerobatic practice area.


An aerobatic practice area may be established over an airport if coordinated with airport management, and can be used to practice landings like those that Bob Hoover does.

Besides the special rules in these areas, it is also a good way to warn other traffic that aerobatics could be taking place because they are listed in the A/FD and a NOTAM is issued whenever they are hot.