Oriental Adventures has the Shikomi-zue, which is basically a sword cane.
it costs 12g, Med. damage is 1d8, x3 crit, piercing.
Tell your GM, "This, but I'm sticking an umbrella on the other end" and call it a day.
Or are you talking about actually beating people over the head with an actual umbrella?
If so, sorry, can't help you there.