Can you replace just 1/3 of a staircase?
 

I would like to create a bend in my staircase that is otherwise just straight. Is this possible and how much (approximation is fine) would it cost to get it done?

Anything is possible but, you want to be real carefull about tripping hazard, I think you should have a landing between straight and curved or at a kink. You should check with the city building inspectors.

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Where can I find staircases that I could use to replace the bottom 1/3? I would like it to be curved... would this be possible?

We build curved in place, you should be able to find hi-end people that could build it for you. Check this out.
http://www.royaloakstair.ca/