If you want a budget-friendly wing option that also functions very well and efficiently, look up Nine Lives Racing. They make great options and should work with you on mounting if they don't have an existing solution.
Edit: looks like they have one for the 128i already
https://9livesracing.com/collections...-kit-97-06-e46
Your best bet for a splitter would be homemade. I used birch plywood for my old race car with some homemade mounts and thin aluminum air dam and it worked perfectly.