Available from Eric Jackson from eBay
or his website.
He also has one die proof available on his website, "Canada Beaver design, die proof in green, india die sunk on 152x229mm card, American Bank Note Co. impirnt and die number 51948."
If you search eBay
with "EKKO" in "stamps" there are currently over 50 for sale. In the completed history 231 sold in the last 90 days. Including for $1200.00 a two volume album set with one 1924 EKKO Broadcasting Station Stamp Album with 99 EKKO stamps and a second Radio Stamp Album 1925 published by Des Moines Publishing Company with 221 stamps, EKKO and other wise.
The EKKO stamps can be found on postally used covers as an added label.
More information can be found here: http://goscf.com/t/8278
[Edit: Jim, you're welcome. VVV ]