Can I use Heavy Ball to look for a LEGEND Pokemon half?
No, you cannot. The Retreat Cost is only printed on one half of the LEGEND Pokemon. If it is printed on both cards, then it is a common attribute for both pieces; but since it is printed on only one half it only applies to the LEGEND Pokemon when both pieces are in play.
Source: TPCi Rules Team (2012-04-05)

Relevant cards

Note: All applicable cards may not be listed below.
BW4_88
XY8_140