Some grocery stores will carry it - in the pharmacy section, sometimes in the tummy treatment aisle...call ahead. If you burn only clean hardwood, then the charcoal from your woodstove is probably (I cannot guarantee anything) fine - just grind it up into a fine powder.
Sometimes with enough...
She could have picked up botulism from the temporary pond. Is the water kind of icky?
I would offer her water with food grade activated charcoal in it. It absorbs toxins. Probably a tablespoon charcoal in a quart of water - let her have that for a while to drink, then switch back to fresh...
EYP is not contagious. A dog and cat vet can have the sample tested for parasites.
You might also convince them to check for weird bacteria - they may be able to look under a microscope. Or have the lab check. There are certain groups of bacteria that are considered unhelpful - they may be...
That's how my duck acted when she had egg yolk peritonitis. If you can get a vet's help, that's best - it is a systemic infection caused by yolk getting loose in the body cavity.
It is also possible she may have a virus, a bacterial infection, or parasites. Might also be some kind of toxin...