Normally lead screws comes in meter lengths and some retailers may cut the lead screw to size and chamfer the ends.
I think what you mean is to build a reprap with print volume of at least 16 inch on all sides. The source code for the firmware is open source and is available online.
Lead screws are intermediate level parts for repraps, it will be expensive but you should see a big difference in print quality. You can go pro with ball screws and its even more expensive.