I suggest Pre-cat O2 sensors as these are really what monitors the fuel mixture as well as monitors the SAP system.
You want Bosch Direct Fit O2 sensors, you do not want to buy them from the dealer. Using online coupons from AutoZone, Advance, Amazon or other suppliers, 4 wire, non Wideband O2 sensors should be between $40-$45 each if you can find decent pricing.
Rock Auto is worth checking as well.
You might try the BMW Pricing Tool,
BMW Part Number Price Comparison for Windows & Mac and see if this will work with the Bosch part numbers.
Verify the Bosch part number using this site -
https://www.boschautoparts.com/en/