Lab glass suppliers online. Be sure to look around, many of them are horribly overpriced or even cheap glass sold as borosilicate!
As far as I know, those little vials (I use the smaller ~1" vials rather than the larger ones the vaporizer includes for ease of use) all use a very similar thread pattern. I did not look up or measure the pitch and length of the threads, but a simple visual comparison was just fine. Just don't screw the vials in super tight (just twist until it sits in there steadily) and you should be gravy.
A tip on breaking through:
It will take a bit of time to heat the glass to near vape-temp, and you should lightly inhale the whole time to prevent burning... Once it comes close to vaporizing (you will be able to tell by the feel of the air in your mouth/throat), calmly finish your breath and exhale. Once your lungs are empty again, go back to gently heating the vial and inhaling... Once the DMT starts to vape (it will bubble in the vial), give it just a tiny bit more heat and back the flame off while still inhaling. Once you get it up to temp, the DMT will keep vaporizing for about 10 seconds without any heat. You can always give it a few more gentle kisses of flame to keep it going.
Unless you clear your lung just before it starts vaporizing, your first hit will be very small. This can be tricky for a bang-zoom breakthrough... but it is actually very nice for a small "chill out" hit before the ride.
The thing takes a bit of practice to get the hang of, but in my experience less so than the GVG. It's also very very easy and intuitive to use once you do learn it, and can be hit easily and effectively with Eyes Wide Shut.
Great purchase, I hope you enjoy it!
Happy Travels!