Most of the ones I've seen just scrape other job sites or feeds, like Monster or Indeed. Standalone ones are going to provide higher quality listings, like:

Authentic Jobs, although you have to provide search terms: https://authenticjobs.com/#onlyremote=1&search=ux%20or%20ui%...

We Work Remotely, formerly the 37signals job board: https://weworkremotely.com/categories/1/jobs

Slack at Work, for companies that use Slack, although you seem to have to search for terms one at a time: http://slackatwork.com/