Buy new, they are very cheap and the price of skimping on valve locks is not worth the possibility of a dropped valve that will wreck the motor. You can also replace valve seals while in there which are very cheap. If you are noticing a puff of blue smoke when you first start the motor its from the valve seals letting oil get past when the motor is off and when you first start it up, the motor burns off the oil.

 

Ok cool! I don't see any smoke ever, but I think i'll replace them anyways

How do I replace them?