If you don't have the BMW ETK/EPC software that contains the "spring table"...you'll need to contact a BMW dealership's parts department...give them your VIN (and options). They will input that into the BMW ETK/EPC and can then give you the 11 digit BMW part number for the coil spring that matches your VIN & options "spring rate"
Once you have the 11 digit BMW part number...you then can probably find comparable non-BMW coil spring options. BTW...the parts department and service department are different areas. You can call or stop by your local BMW dealership's parts department and get price quotes for parts anytime you need them without making an appointment for service work (or the cost of using their service department.
If you click the following link to the BMW TIS...you will see that it mentions basically what I wrote above. BMW uses a "spring table" to determine which coil spring:
https://www.newtis.info/tisv2/a/en/e...unting/OQ2jHrW
The pics below shows an example of how the "spring table" is accessed: (my 97 e38 was used to pull this info up years ago)