You can use an addon I think there are few if you search em.
Or you can bower instal bootstrap and then in your brocfile app.import the bits you want directly from your local bower folder. IF you want to use less or sass you can also link to the css using import in your app.less / app.sass
I’m using only the affix javascript here is where I import it in my brocfile
[
'bootstrap/js/affix.js'
].forEach(function(i){
app.import(app.bowerDirectory + '/' + i);
});
Including css in my app.less
@import '../../bower_components/bootstrap/less/bootstrap.less';
see also