You can use any type of bowl, even a large coffee mug, as long as its deep enough
to hold the lather and wide enough to allow the brush to swirl around.

And yes....bowl lathering is a bit more time consuming, but the lather probably will
be thicker and smoother if you make it properly. Try it for yourself, using whatever
bowl is at hand, and see if you like it better than face lathering. Then you can decide
if its worth it to you to get a "real" lathering mug.