2 secs of googling revealed this link:
Congressman Admits Taking Bribes, Resigns - washingtonpost.com

I could dig up more, still I'm sure you can do it yourself, just type 'bribes in congress' in the search field.
And I should probably point out that this scandal hapenned because somebody got actually caught. They don't take bribes and announce the fact from the highest balcony in Washington, so I'm practically sure that the practice exists.