There are 3 double bells for sale right now on eBay:
The nicest one is restored 1929 Beuscher, beautiful, and costs $4,150:
ITEM SOLD FOR $4,150
https://www.ebay.com/itm/BUESCHER-DOUBLE-BELL-Euphonium-Baritone-horn-RESTORED-Orig-silver-finish-1929/392119254332
There is another Beuscher from 1925, in original used condition as far as I can tell, for $1,700:
ITEM SOLD FOR $1,600
https://www.ebay.com/itm/Buescher-Double-Bell-Euphonium/292854251816
And there is a Conn that looks in decent condition, and may be a converted high-pitch horn based on the photo, which is over-priced at $4,500, but has a make-offer button:
BIDDING HAS ENDED ON THIS. NOT SURE IF IT SOLD.
https://www.ebay.com/itm/Euphonium-Conn-Double-bell/143021299515