I can't really answer the "who should I use" part of the question (as it's off-topic), but given that I do have significant experience in making sites/applications scale for high-traffic loads, I can definitely suggest that you look at getting a Reverse-Proxy CDN.
The last company I worked for used Yottaa for this, and were able to use their services to survive some absolutely huge traffic spikes generated by TV advertising.
There are lots of CDN services available, but you'll probably see the most benefit from one with an "origin pull" mechanism, where user's requests hit them first, and they request the page from you once, and then all further hits for that document are served by the CDN's servers.
You'll do best to pick a half dozen different CDN providers, and then set them off against each other, and see who can battle the best deal for your requirements. Big CDNs like Akamai are probably gonna be insanely expensive for your scale/needs.
I'd mostly forgotten that there's actually some Free CDN services from the Coral CDN, and Cloudflare (free to a point, I seem to recall).
One of the intriguing things that some CDNs can provide now is SSL acceleration, where they serve the secure portion of your site with a Subject Alternative Name certificate they generate, so that they can "pretend to be you" in terms of what the users see.