Most quality sites should tell you what lowering springs will drop you. I'm pretty sure American Muscle lists it for every set of springs they have. The ones johnny_raincloud linked to lower more than most other springs. Sportlines likely WILL cause you trouble on speed bumps and driveways. The Eibach Pro-Kits give a little less drop, around that 1.5" mark. I'd suggest that with just about any lowering springs and aftermarket exhaust, you may find certain speed bumps that catch you. I even hit my prochamber sometimes at stock height if I hit some speed bumps too hard. But yeah, like johnny said, check AM and look at pics as well.
If you just want around 1" drop, you may just take a look at the Ford Racing C Springs. The BBKs look like they about 1.25". A lot of the rest are around 1.5".
2006 Mustang GT
2008 BMW E92 335i - GONE!
2003 Mustang Cobra - GONE!
1997 Mustang GT - GONE!