Not saying coilovers aren't the way to go; just thought I'd add a bit more information to the search for stock springs...
The Sachs spring catalog (wish I'd found it sooner!) has the BMW spring code referenced against their part numbers:
https://drive.google.com/file/d/1Of8...ew?usp=sharing
One useful nugget of info is that front spring codes D7 and D8 both refer to the same Sachs part number (993 257). I think this is decent evidence that D7 and D8 springs are actually the same part with different labels, even from BMW.
I'll search the rear spring codes soon to see if I find anything useful for them, since their coding seems to be so random for e91s...