It is difficult to find: VIN 113044, Group 82, SA 55502, part number A 000 544 58 32 26. Needless to say it is expensive, $230 or so.
What may happen is, if this Japan blinker relay is designed for additional 2x18W bulbs, it will not work properly with additional 2x 2W bulbs. You would have to put higher power bulbs in side blinkers than 2W.
I have not done it, but I would try first having 18W bulbs in regular blinkers (not 21W that you may have) + 2W in side blinkers and see how it works. But it is not responsible advise as I have not tried it. It also may put too much of load on blinker relay longer term. There is no specific blinker relay for Italian 2W side blinkers.
Another way forward would be to use generic blinker relay, not a Mercedes one.