Progressive Suspension 444 HD is what I got for mine. The HD model is for 250 lbs and up, though they have one model now apparently with even stiffer springs (at least their web site quotes three models).
I'm very pleased with them, they swallow all the small bumps with barely a bump felt, and do a decent job of the big stuff also. The bike handles perfectly in the twisties also even with a full travel load. And they're hand adjustable, you grab the entire spring and turn it, it's threaded and is something like 12 turns between minimum and maximum preload.
Price wise, not outrageous; JC Motors (or just hit up Amazon) has them for $500-something.
For optimal results you probably also need to address your front shocks. Progressive makes springs that might help. There are also cartridge stuff or emulators, not sure what's best there.