I would think if you have to take off to replace the old one, why not take it to a lock smith and have it rekeyed?
I think it would save me downtime, and probably money, to just order a cylinder/key on-line and install it myself. And if I take everything to a locksmith, I have to remove the entire assembly (rather then just the cylinder). Also, I would think the bill would be at least $50 to go to a locksmith (does anyone do anything for less than $50 these days?).
I don't know what a cylinder/key costs but I would be surprised if it's over $25 with shipping.
But if I can't buy a cylinder/key, then the locksmith is my only choice.