You could also just buy a bottle of light Karo and use that instead in a pinch.
Noooo! Don't do it! There are other flavorings in Karo, like vanilla extract or something, that you don't want in your mojitos.
At a Japanese supermarket, you can buy a bag of tiny containers of simple syrup. They are the same kinds of little individual serving plastic containers that coffee creamer comes in and that's what it's for, to put in iced coffee. I confess have a bag of those - they were originally intended to bring to work, but now I basically don't go to work - and I use them if I am really lazy.
I was very surprised to see these containers of syrup at coffee shops in Japan and when I mentioned to my Japanese friend that we didn't have them, she was quite bewildered. She asked what did we use in iced coffee then, and I said, just regular sugar. She said, but that doesn't dissolve! I said, well, yeah. She was shocked to hear that we would put up with such a low standard of iced coffee sweetening.