The first question is: why do you want a new slide?
Just to have a second one?
Or did something bad happen to the original?
If it's to replace a broken slide, Glock has REALLY good warranty policies. Some friends of mine just got their slides replaced for free when they broke. Another, who wasn't the original owner, got his slide replaced for a REALLY good price.....much cheaper than you could ever buy one anywhere else. If that's the case, PM me and I'll get you the email of the Glock Canada rep that they dealt with. The caveat being that you have to send the old slide in and you don't get it back. They're not a normally purchasable item from Glock.
If you lost it......well, I'm not sure how that would work with Glock.