We have been using Euca-Mint for 8 years for respiratory for both Budgies and my self For asthma with good results. We have several books on the subject, but have not found the right one yet. I get lost in the details. I support all of the oils listed by Kate C as safe. We use electric crock pots as they last 8+- hours for treatments with good results.
We have a human nebulizer to produce deep penetration for respiratory issues. Per our Vets instructions We attach a mouth piece between bars on a small hospital cage and add meds and solutions per vets instructions just as the vet does in his office. It is great to treat several birds at once or only one as needed. We use a separate mouth piece for birds only and sterilize after each use. Oils can clog a nebulizer, but work great in an inexpensive crock pot or an old fashion vaporizer.
I was glad to see that Vicks uses eucalyptus etc. my only caution is use only a few drops of the essential oils but not the Vicks for the birds, just in case. There are several homeopathic meds like Milk Thistle and dandilion or Pau D'arc and others that address serious problems that regular meds do not cover well. We use milk thistle /no alcohol/ on a regular basis and have some experience with some others, but not enough to
state definitive results over a number of trials. ladyguldianfinch.com carries bird approved selections. they do ship international.
A good local health food store should be a good source and ours has a whole section and books for humans. Amazon has a number of books available. there are a number of vet text books available on the net which help with meds and titrations for birds and exotic. we have the same one that our vet has so we can discuss alternatives and known meds for special issues and some research like using Sodium Benzoate for AGY in conjunction with Amphoteracin B or in higher doses alone.
Best wishes, Jo Ann