I think I mentioned this to you in your build thread before you bought it (because I looked into getting one myself):
Places that fill CO2 I do not think are going to fill a metric bottle, which was my main concern even if there was a CGA adapter to metric. Safety reasons probably, the whole valve would need to be swapped before being filled.
Have you tried fitting a smaller tank like a 24 oz paintball canister in there?
If not I can't think of any solution. You could use the ADA disposable CO2 Advanced kit, that looks just as cool as the Tower I think.
You have to remember ADA is a Japanese company first and foremost that creates products for the Japanese market. Then the international market. That is probably the reason why no one in North America has one.