Freeze-Off made by CRC is what I used on the last stuck seat post I had.
After soaking in penetrating oil and all the tugging on an attached seat I could do, I sprayed this stuff on it and the post started moving in 5 minutes. The Freeze-Off seemed to make the parts cold as I sprayed it on, and supposedly that makes the different metals move different amounts. Not sure if that is why it worked, but I was very happy it did.
Bought it at the car parts store. And I have been using it on all my stuck/rusty car parts as well.
Good Luck