Are the threading for supressors all pretty much the same? Is that the only reason you would need a threaded barrel?
Threaded barrels allow you to mount suppressors, though a separate adapter may be needed depending on the screw dimensions. The barrel has to be so wide to allow the bullet to exit. Movies tend to show internally-threaded barrels, this is much less common in reality with centerfire pistols.
Where can supressors be purchased? Or do you just make your own?
The short answer is that suppressors can be purchased through a Class III FFL. They can also sell short-barreled rifles/shotguns, full-auto weapons, etc. As mentioned, you need to send a form to the ATF and pay a $200 tax stamp.
Apparently you can make your own in Kansas. The state passed
SB 102 last year that offers legal protection for private manufacturing (only inside the state), and there are DIY guides out there. But purchased suppressors are of much higher quality.
To my understanding, they are only illegal if attached to a gun?
This question doesn't even make sense.
Will it affect the warranty?
Most likely not, but I'd look at Glock's warranty terms. If you go the purchase route, you would likely only have a few months as Glock only has a 1-year limited warranty.
(As always, double-check laws and info for yourself)