Google “Search Term site:username.tumblr.com” (or site:domain-where-your-tumblr-is-hosted.com). Example.

I’ve been doing this for awhile, but I get the idea from how often others on my dashboard complain about Tumblr’s non-functional search that a lot of people don’t know or use this workaround. It would be great if the in-site search ever worked, but in the meantime, the Google way is helpful for finding old stuff from your own archives as well as that one thing you know you saw that one time on someone else’s tumblr and are desperately trying to find again.