There are no rules for buying or HIRING a giant, but I know the Albion campaign used them as random happenings.
Mordheimer's site also had the scenario, "Don't Wake the Giant"
Also, the Storm of Magic campaign I've been publishing (Untested at the moment), has the same Young Giant as one of the possible monsters that you run up against.
The "Advanced Scenarios" which include the Young Giant are not in pdf form at this moment, but are found on another thread: http://boringmordheimforum.forumieren.com/t5456p25-storm-of-magic-mordheim
Hope those help!