How do I search for channel names in WeeChat?

Use the /list command... the standard IRC command for listing and searching IRC channels.
You can add a regular expression at the end to search for specific chans.

You can use the /list -re <regex> option to search with regex.

If you need more info, use the /help command; and learn more about real irc functionality (instead of relying on GUI wrappers).

WeeChat doesn't have a channel search feature, as far as I know. But however, you can use the ALIS (Advanced LIsting Service) channel bot (on freenode):

To search for channels containing python in their channel name:

/msg alis list *python*

To search for a channel whose topic contains the word foo:

/msg alis list * -topic *python*

(the wildcards are required)

For other options, see:

/msg alis help list

/list works too, but on a big network like freenode (with over 40 thousand channels), it may flood your client off with the results or produce unexpected results.

Note: This is a network specific feature of freenode, and is client-independent. It can be used from any IRC client.