you can use the wildcard search on the dashboard to find caches
What keyword to use depends on if your challenge is for the placed by name, or the real co name (as they can be different).
Here is an example for finding owners starting with 6 in sweden
Sweden @placed_by 6* or Sweden @profile_name 6*
Basicly do country, region or county first, then keyword, then search string
Unfortunately it looks like search will return all owners containing 6 and not starting with it, but its easy to sort the column by clicking on the header