Lots of the usual vendors sell them (assuming GMC's are the same as Chevys.)
Here's one but you'd also need to buy the cylinder and key separately.

Sent you a PM

Kevin
1951 Chevy 3100 work truck
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Photos
1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.