Back when I had my B-40 I was able to buy one of the B-40 type speaker from Allied electronics.
Steve

I have used the B40 as a back up for at least 14 years and I agree with the last poster that the little speaker is fine. Like the manual says you can fit a bigger 8 ohm speaker but with increased current draw. I am actually trying to find a spare B40 speaker so if you decide to change yours I will buy it from you!

http://www.borgeltinstruments.com/B40man.pdf