This is a great question and a subject that probably deserves its own video! I haven’t done any real research into tube quality yet. Usually going with the company that sold the scooter is the best bet to get something where the stem angle is going to fit, but since you’ve tried that, the next best thing would be to take out the tube you have to see if there are any part numbers on it (to find the exact same tube). I don’t know if reputable companies like IRC, Pirelli, Michelin, make tubes for our scooters or not yet. I would see if Cheng Shin Tire (aka CST) makes one that fits. They’re not as high end as the other brands, but I’ve used their motorcycle tires on the freeway, and have seen them hold up for years of hard use. So, they at least know how to make DOT approved tires and tubes :-)
Let us know if you learn anything more. It’s something I’d really like to know more about.
-Paul from ESG