Have you not got a mid or U.S. bb knocking about? Quick measure up and you're there.
A normal mid/U.S. bearing is an R12 2RS. This is imperial, the internal diameter is actually 3/4" or 19.05mm. You could bowl into your bearing house and ask for the metric equivalent of an R12 with a 20mm ID I suppose.
They might look at you a bit funny....