is there a search function for folders in online outlook?

No there is no direct way of searching for a specific folder in OWA.

One possible workaround is to open any email you have and then use the Move to option. This shows a list of folders which should hopefully show you the folder. If you have a lot of folders then you can alternatively use the search field at the top to search for the folder instead. Unfortunately it doesn't show the full path to that folder but it is better than nothing.