Springs. It's not that simple. Generally lowering springs are stiffer too, so they will compress less than stock springs so they might be much shorter, uncompressed. Some guys can often ID them by color, I don't know them, but some do. If they're like light blue, or dark blue or whatever, you might either be able to find it by searching or maybe someone will chime in with what they probably are. There's also progressive springs which make it even more complicated. But the color might be a key factor.
And BTW, not everyone uses it, but the best way to check for ride height is to measure from the fender arch opening to the center of the wheel. That eliminates variables like wheel size, tire pressure, tire size etc.
An inexpensive way to get a little lowering on the front end is to replace the spring perch (the hat shaped piece that sits on top of the spring) with the spring perch from an e90 3 series (2005-2013). On my car I think it went down a little over 1/2 inch. I don't recall the part number but there should be a few discussions here about how to go about it, use the search function at the top of the page to look for them.
Be careful about going too low, knocking a hole in the cast aluminum oil pan would ruin your day.