Found these 15 inch Carina GT-R rims (also featured on the Celica Supra and Soarer) on auctions.yahoo.co.jp which look exactly like mine:
Immaculate Carina GT-R rims on auctions.yahoo.co.jp
These rims are 15 inch, ET +20 so they are actually exactly the same as mine! I took the gamble that the 15 inch Celica Supra rims would be the same size/offset as the Carina and apparently I guessed right!
They cost about 37000 yen (300 euros) so I think the price of these rims are spot on!