Can I buy a soda at a Japanese takeout restaurant?

I guess this depends on which country the restaurant is. It's always assumed that in the big cities of the United States you can always get any item of food or drink you want, and soda being popular, it's likely that even their Japanese restaurants can supply this.

Outside of the US, it might be sensible to expect Japanese food and drink in Japan, and if you seek a Japanese restaurant, the expectation would be that you wish for their own food and drink, not American ones. I guess you can get soda in their large supermarkets and that there may be tolerance for restaurant customers bringing in their own drink, if, like medicine, they cannot contemplate a meal without their soda.