This article is from the Recreational Figure Skating FAQ, by Karen Bryden with numerous contributions by others.
The boot must support you, otherwise you will be expending most of
your energy just holding your ankles straight. See about worn out
skates. Grasp the boot by the top of the ankle and hold it sideways
(parallel to the floor). If it droops, it will not provide you the
ankle support you need. Don't buy it. Look at the condition of the
boot - it should be leather and not some kind of plastic or pseudo
leather with a cloth lining. There should be no cracks or tears in the
leather, though some creases are fine.
Your best bet is to check any rinks in your area - see if the skate
shop, rink office or pros/instructors have any used boots for sale. If
there is a bulletin board or skate club, check any advertisements or
advertise that you are looking for size-N skates.