How can you go back and buy a Founding Father...

[Zubbus]

...after you said no initially?

 

[dalgo]

The manual says that if you refuse a founding father he will never offer to join you again, and that is what I have found in my own games.

 

[schlaufuchs]

You can't go back. That's why it's really important to pick and choose carefully. A lot of people like to decide from the beginning what fathers they want to go for.