Tumblr URL Checker
This tool provides a quick way to check a lot of Tumblr URLs as to whether or not they are valid. View it online here
How to Use
Paste a list of desired URL subdomain prefixes to check separated by either commas or line breaks, and hit check. The tool will query each URL to check whether or not it exists and report back the ones that do not currently exist. Sometimes, a URL will not exist, but also not be registerable. I am not sure how to address these false positives as of now.
The method here to fetch pages could be repurposed to support more than just Tumblr urls.