Depends on what you want to do...
There are many manufacturers of turnouts and flex track. Atlas is probably the most common, but Peco makes excellent turnouts, and the Walthers-Shinohara ones look the best in my opinion. MicroEngineering also has great looking track and turnouts.
Then you also have to consider what code - Code 100 is usually less expensive than Code83, but in my opinion, Code 83 looks better. (My personal choice is Walthers-Shinohara turnouts with Atlas flex, in Code 83.)
Prices vary, but your best bet is probably an on-line place for lowest price. Be aware that you will then also pay shipping, and also run the risk of not getting something right because you can't see the stuff in person before you pay.
But first things first - do you have a plan? Nothing is more expensive than something you don't need, no matter how low the price.
Is there anything else you'd rather have, or also need instead or in addition to track and turnouts?
Andrew