If you just want to use it, then I recommend making use of a CDN. For example, head over to cdnjs.com and you can find it there and add the reference to your page.
Direct Link: http://cdnjs.com/libraries/bootstrap3-dialog
<head>
<link rel="stylesheet" type="text/css" href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap3-dialog/1.34.5/css/bootstrap-dialog.min.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap3-dialog/1.34.5/js/bootstrap-dialog.min.js"></script>
</head>
You may also open the development/production js file directly on your browser and save it from there.
Or you can install NPM, then with NPM install Bower, with bower you can manage your assets much easier. For example, you can run bower install bootstrap3-dialog
and this will take care of downloading everything for you. Then you pick whichever you wish to use "development" or "production" css/js files.