Talk to Penelope Redd again. She will tell you she knows a rich fool who is selling it. Talk to Luthor and pay him 300 Simoleans. Check your mail, then pick up the plutonium behind City Hall.
I have tested this and it works. Just go to the Blackout! episode and instead of giving the 10 Nuclear Rods to Mamma Hogg, save and leave the episode. Then you can sell the 10 rods in a different episode! Do this 3 or 4 times to get tons of cash!
Talk to Penelope Redd. She will tell you to talk to someone who works around water. Then, talk to Old Salty.
Talk to Optimum Alfred in the Optimum's Tech Factory. He will make you play the chop shop game. You will need to find someone strong enough to rip out iron shelves. Talk to Jimmy the Neck and he will do it for you.