was going to suggest the Nuprol retention holster but I think they only do the right handed variant
actual G17 for real steel will not fit straight off the bat and will require some work (not recommended)
IMI Defence for Glocks .45ACP i.e. Glock 21 will fit but has some wobble. Again sourcing a left handed version may prove difficult
Otherwise these might be an option
http://www.uktactical.com/c-787-new-lefty-uph-holster-warrior.aspx