These type are the ones you need, If you want a set of locking ones, I have a set of 4 high quality ones for £15 posted.
http://www.ebay.co.uk/itm/Chrome-Wheel-Nuts-x-20-12x1-5-Lexus-GS-LS-IS-IS200-GS300-LS400-SOARER-/120839307320?pt=UK_Cars_Parts_Vehicles_Wheels_tyres_Trim_Nuts_ET&fits=Car+Make%3ALexus&hash=item1c22958038