checl with these guys - they somehow stock a whole lots of tubes - but do check with quality first - they are known to send fake ones though (which they sheepishly agree to change once they know you know what you are buying). Check with them first
I have bought quite a few Valves ( 12AU7) from BE3 amplifiers . Once he sent me a Valve which was not working , After I bought this issue to his notice , He immediately replaced it with another valve. You can contact BE3 , he should be able to help you.
His number is 098202 23719
ECH81 is used in mixer circuits of a radio. But do you really need that tube? Is it physically broken? It is suppose to last a lifetime under normal operation. Even a lightening strike or two shouldnt do any damage.
Due to its location in the circuit these mixer tubes are run very benignly. Usually there are other tubes (or things like caps, resistors besides other tubes) which will break before an ECH81. In my opinion you should check again.
These tubes were made in truckloads by BEL. Since they have no audio application, they go pretty cheap. Ask around some old radio shop. They may have them.