it should be

Hydrogen or

helium

Hydrogen: flammable and dangerous. Advantages are: low price.

You can be more localized.

Wedding etiquette

The company will look for it, but they will charge a high price.

Or, if you are in the street, square or park, turn around.

I'm sure I can find some to sell.

Cartoon hydrogen balloon

A peddler. You talk to them, and you can get them for a little money.

Helium: Helium is not dangerous.

Disadvantages: more expensive.

Helium. To be honest, it's more troublesome. Their wedding etiquette company will have a special one here.

Replenishment channel

. But these buying channels. If you want a can or two, people won't sell it to you.

If you can buy it online, you can find it on Taobao or buy it.