What large chainrings are compatible with Shimano FC6603 crankset? On my race bike I have a crankset FC6603. I want to replace my outer chain ring (My inner and middle chainrings are OK).

But I cannot find an online store where they sell the outer chainring of this series. The middle and inner ring are available on the online stores, but not the outer chainring.

Maybe the outer ring isn't be produced anymore?
Which series (with an available outer chain ring) is compatible with the series FC6603?
The FC6603 seems to use a standard 130 mm BCD 52t chainring for the outer ring, unlike the special ones used on some other Ultegra/Dura-ace cranksets. 

You can buy such rings from  most major brands. Look for ones marked 10 speed or 11 speed.