LT1 Evap Canister Code

I have a code that the mechanic says he needs to run a smoker through the system to find the leak. I will replace the fuel cap just in case. The charcoal canister was replaced recently. Is this an expensive fix? How could I find the leak?

Update - I found the Evap description in the Silverado group. Also, a misfire in the #7 plug wire was found and fixed by replacing the plug wire. Unfortunately, I had to purchase a complete set of plug wires. Does anyone sell single plug wires? This seems to be a recurring problem with the LT1 plug wire due to routing near the manifold.