The Sub Battery (commonly called AUX Battery) is a 12AH AGM power sports (think motorcycle/ATV) battery.
If you have an Interstate Batteries distributor in your area, I'd remove the old battery and bring it to them. They'll buy your old battery for scrap and sell you a quality replacement for a reasonable price. I believe the Interstate MTX-AUX14 fits, and it's around $120.
If you want the genuine Mercedes replacement, (part # 0009829608), then FCP Euro sells it for $142.99, and they will ship it to you.
Alternatively, you should be able to find a suitable replacement at any auto parts store that carries motorcycle batteries. Even many Walmart locations carry them.