you can get it on eBay, but probably not worth paying the price.
IC Diamond is known to be abrasive and actually scratch chip packages - this can make subsequent TIM applications less efficient (unless you mirror-hone the surface each time) and can make reselling parts with "damaged" and worn-looking factory markings very difficult.
Gelid GC-Extreme has low viscosity, it's thin and runny. It seems second to none for extreme sub-zero cooling, but it does gradually "burn off" over normal use and requires comparatively frequent (maybe even seasonal) reapplication. Prolimatech PK-3 is a remarkably similar product (in terms of actual usage). Other top thermal pastes are Phobya HeGrease and Tuniq TX-4, I have no experience with either.
There are, of course, even better products like Indigo Xtreme and the Coollaboratory Ultras. These are expensive and highly sensitive to application (highly variable performance). They are actual chunks of metal, not metal-bearing pastes, which require a temperature reflow procedure to install. And they often require a lot of mechanically subtractive patience to fully remove.
There are dozens of websites dedicated to authoritative TIM measuring, testing, and comparisons. And very few agree on their top picks, although the same handful of products (mentioned above) tend to always appear in the top-tier slots. OC-enthusiast sites are filled with combative diehards who will defend the honour of their chosen TIM with endless fervour.
I personally use venerable Arctic Silver 5 for these sorts of applications. It may not be quite as efficient as other products but it consistently ranks high up in the lists and is a steadfast, proven performer (it's also cheap and readily available). And - unlike most extreme TIM products - it's a throwback from the days when TIMs were engineered to last "forever" instead of being reapplied with great frequency. Reviewers and benchmarkers and other online "laboratories" excitedly swap out their parts every week as new parts roll in for testing - they are hardly aware of TIM longevity or degradation over time - other people (like me) tend to build platforms which last and upgrade/swap our parts on a more annual basis.