She sounds like she definitely isn't feeling well. You know, different vets charge different rates for different things, so there might actually be a vet out there that could help you for a little more than the antibiotics would cost.
Otherwise: If it's cold out side it couldn't hurt to get her some place warm until she is feeling better, and better yet away from your other ducks until she is better.
Without knowing exactly what is wrong it is going to be hard to know how to treat her but you might be able to find some broad spectrum antibiotics at your local feed store.
Beyond that I'm not much help.
So overview, Someplace warm quiet and isolated. Make
certain vet help is beyond your means. Broadspectrum antibiotics. Good healthy food. (I added that last part

)
Edited to add: Sneezing itself isn't so much a big concern if she has only done it once or twice but lack of energy is always something to be concerned about.