Even though the tread pattern isn't an exact match between the Proxes ST and the Proxes 4 tires, the sidewalls and tread blocks are nearly an exact match when viewed in profile, from the side of the vehicle.
This is the best price I found (including shipping) when I ordered my extra set:
http://www.prostreetonline.com/x/toyo-proxes-st-tires.asp ($143 ea, 15% off with HD5T coupon code + $16.86 UPS ground shipping - I just ordered an extra pair)
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