You're not gonna find OEM parts easily. Is just the sub blown or are the other speakers going as well?
Just replacing the sub is an easy project and can be done in under an hour. The proper way to access the sub is to remove rear panel around the speaker which requires removing the seatbelt bolt on the rear passenger seat, double decker cargo system and then a few clips. You can also take the shortcut and simply cut the mesh surrounding the sub enclosure to access the sub using this method:
https://www.youtube.com/watch?v=HjuK3WjJngQ
The MB Quart DS1 is recommended on here often but I ended up going with a Kenwood KFC XW800F.