You will probably need a subscription service like fonts.com that allows you to embed non-standard fonts on your website. Google Fonts (which is free) is also a possibility, but they don't provide Arial Narrow. There's a good chance you'd be able to find something similar, though. I use GF for our own landing pages and it's super simple.
There is also the option of uploading your font to your website and coding the stylesheets yourself:
http://www.techrepublic.com/blog/web-designer/embed-custom-fonts-on-your-website-with-the-font-face-property/
That said, I've been able to do this with websites, but haven't tested it with email. With all the additional coding and CSS, many email providers may strip out the formatting or mark it as spam (my biggest fear!)