GMC Repair: key replacement, powertrain control, horn blowing

Question
We bought a 2000 Yukon, used, and only got one set of keys.  We have tried (twice)to buy a second set, but each key makes the alarm sound.  Now we have lost the only set we had (they are around here somewhere) but I need to get the truck moving.  Why is the dealer unable to help with the replacement?  If all else fails, how do I disable the alarm so I can use the replacement key(s)?

Answer
Hi
if possible, the alarm, what does it sound like? Just the horn blowing?
Or is it an aftermarket alarm.
That system does not use a special key, but a sensor that sends a valid code to the BCM (body control module) and PCM (Powertrain Control Module).
Please follow post back
Pawl