Well, if your spring rate is already high (as you described from the stiffness over bumps) the stiffer anti-roll bars (if you even have them at all) will help the car in roll. When both wheels of one axle bump together, the anti-roll bar has no effect on the system, but in roll they do. Since you said there are no coilover systems available for your car, it's probably pretty unlikely that there are any aftermarket anti-roll bar available, too. How good are your fabrication and engineering skills?

If you just want to lower it, you can simply buy new springs. If you can't find any aftermarket springs, you may need to do some measurements and calculations to come up with what you need. If you do it right, you can get exactly what you want in a custom order. It might cost you a little more, but you'll be the only one with them!