Are STOL kits that use leading edge slats available for 'typical' GA aircraft?
Lowering stall speed is not a benefit in itself, it's only a benefit if it you need it. There are STOL kits for many airplanes, you don't see them much on most airplanes because there's no real benefits in normal flying and they cost a fair chunk of money to install. STOL kits only get you so far, adding a STOL kit onto an airplane with a 50 kt stall speed will not give you a 25kt stall speed, it's a lot less than that. If you have a bush airplane, fly into very short fields or in high density altitudes then a STOL kit can give you extra capabilities. If you don't fly into very short fields then a STOL kit is extra weight and costs for capabilities you won't use. 

It's also not as simple as simply sticking some extra bits on the front of the wing, there's many considerations. For one, if you drop an airplane's stall speed substantially then you will run into control authority issues at slow speed. You will have to beef up the ailerons, rudder and elevator to compensate, which is a substantial re-design, adding costs. The most popular STOL kits are for airplanes which already are good STOL aircraft to begin with and have adequate control authority.