If you want to key in a search term and see what channels meet that search criteria, then I think you're out of luck.
However, you can sort of search for channels by going to the Roku Website
, then clicking What's On
, then click All Channels.
From there, you can use the Find capability of your browser (usually CTRL-F) and type in the term.
Note that this will be an exact search only. Also, it is only as up-to-date as the Website. I suspect that channels are added to the store before they're listed on the Website, and if so, I don't know how much of a delay there might be.