A bit pricey, but here:
http://www.ebay.co.uk/itm/Truvativ-Left-Hand-Crank-Omnium-GXP-Super-Aluminium-165mm-170mm-Crank-Length-/361435275330?var=&hash=item542738a042:m:m1PtH7QJRqSB-eZQbSg0g-g
bit cheaper, but not the sizes (worth it asking?):
http://www.ebay.co.uk/itm/Truvativ-Omnium-GXP-Left-Hand-Crank-Arm-167-5mm-172-5mm-175mm-180mm-/331438936769?var=&hash=item4d2b4cd2c1:m:m8ilBRGZjB0SjFyAKzTao4Q
@c4shr started
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A bit pricey, but here:
http://www.ebay.co.uk/itm/Truvativ-Left-Hand-Crank-Omnium-GXP-Super-Aluminium-165mm-170mm-Crank-Length-/361435275330?var=&hash=item542738a042:m:m1PtH7QJRqSB-eZQbSg0g-g
bit cheaper, but not the sizes (worth it asking?):
http://www.ebay.co.uk/itm/Truvativ-Omnium-GXP-Left-Hand-Crank-Arm-167-5mm-172-5mm-175mm-180mm-/331438936769?var=&hash=item4d2b4cd2c1:m:m8ilBRGZjB0SjFyAKzTao4Q