When I'm finding something rare online, this is the process I use:
1. Google Shopping. Pretty straightforward, you search various online websites including eBay all at once. If you see a few results here, it's probably not a rare item after all
The problem is most distros are not included, and even eBay or Amazon are hit-and-miss.
2. Standard google search. Actually this gets complicated, but you're searching almost every imaginable website. Crafting your search properly is of the essence. These days there are a lot of worthless lyrics and MP3 websites, and you probably want to avoid these. Make sure to use quotes (") to find exact phrases and dashes (-) to exclude certain words. The site keyword lets you search within certain domains or websites. For example to find results of Det Frysende Nordariket which are not lyrics or MP3/download and which are from German websites, this is a possible search:
"Det Frysende Nordariket" -lyrics -review -mp3 -download site:de
And you can do a lot more with the advanced search option.
3. Try websites with affiliate programs. Metal Archives and RateYourMusic, for example, have integrated searches for the likes of Amazon, eBay, Gemm, OmegaOrder, and others onto their release pages. The rarest items will probably not be found this way, but it's worth a shot.
4. Search websites specializing in rare music: Gemm, Musicstack, CDAndLP are a few that come to mind.
5. Look for similar, more common items. Sometimes you'll find that Google doesn't reach everywhere, but if you find one Ildjarn CD on some website, why not investigate for the exact one you want?
6. Try again later. Don't assume that because you can't find the item today, you will not find it tomorrow. In the past I was searching for years to find a few certain CDs, and finally I did find them by periodically searching.
7. Otherwise, as others noted, you may have to settle for a digital copy. Occasionally even that is not easy, so you may want to search Google for decent DC hubs, but be prepared to offer a bit of your own collection, "leechers" are not tolerated as a rule. Often you may need to formally request access and register to use the hub.