Hi Brett,
Do you know if other barrel mowers use around the same coupling at all? might pop into lawn bowls and see if they have an old one or parts maybe.
No I don't know any other mower you could get the coupling from but you should be able to find something on eBay that will fit ,I've seen couplings from China with free postage for around $34.
It's probably not worth spending too much to fix this mower because you may find a better mower cheaper than the repair.
$34 isn't too bad to fix your mower though, just check all the diementions before buying.
Cheers
Max.