I don't know of a "specific" resource, like one website that is better than another, but this forum is probably the best source you'll find anyway, as it's full of experienced bird owners who have gone through these things and know first-hand about them, or in my case and the case of a lot of others, we've bred birds for years, or we went to school for some sort of Animal Health/Biochem/Medicine major...
It sounds to me like the vet you've been using is just your very typical "Exotic Vet", which unfortunately in the United States only means that they are a General Vet who is WILLING to see animals other than dogs and cats. An "Exotics" Vet in the US can call themselves that without ANY additional education, training, or experience, they just see any animal who walks through the door usually. So your Vet sees a Ferret, then sees your bird, then sees a Bearded Dragon...that doesn't bode-well for any of their patients, as they know no real specifics.
As far as just handing out Antibiotics like they're candy without even taking a culture, this too is unfortunately very prevalent with Exotics Vets, and I think the reason for this is basically time and money. They want to see as many patients as possible, a lot of them don't have a lot of money, so they hand out Baytril like it's the best thing since sliced-bread...and this is why Baytril is starting to not work against any Bacterial strains at all in Birds, Reptiles, Rodents, etc. It's ridiculous. Think about human doctors today; hardly any of them will just give-out a prescription for an Antibiotic without a culture, blood test, x-ray, etc. anymore like they used to do, because look what happened, we have a ton of what used to be very effective Antibiotics that no-longer work for anything...And that's why. So now they are scared to death to give anyone Antibiotics without absolutely knowing if they need one, and that's how it should be...Look at MRSA and how difficult treating it is. That's what is going to happen in the Veterinary world soon because of exactly what your vet and many others do every day. Plus, they hardly ever suggest a Probiotic along with many doses of Antibiotics to protect against a Secondary-Fungal Infection in the bird/reptile...it's frustrating...
If your vet just doles-out Antibiotics to your bird without confirming a diagnosis, you need a new vet. That's the lazy, irresponsible way of practicing medicine, and if the truth be told you don't need a vet to do that, you can order Baytril online without a prescription from many, many different retailers. If your vet isn't confirming what's wrong with a Culture, X-Ray, Blood Work, etc., then why in the world pay them $100 to look at your bird, literally just look at him, maybe weigh him, and give you a bottle of Baytril that you can buy online for $30? I'M NOT TELLING YOU TO DO THIS, LOL, JUST MAKING A POINT!....
Definitely get your Budgie to a Certified Avian Vet/Avian Specialist, and tell them that he's been wheezing ever since you've had him, that the other vet did a Culture but found no Upper Respiratory Infection, but that they never even did an X-Ray, let alone blood work. Tell them that it gets worse upon exertion, and tell them you're concerned about it. The first thing they'll most-likely do is a regular x-ray, which doesn't cost much, and they'll most likely recommend a regular Nebulizer treatment, depending on what is going on. He may have a Congenital Disease of his lungs, he may have fluid in his lungs, he may have a growth, you don't know. All that you know is that it's not a Bacterial Infection in his Upper Respiratory Tract, which I could have told you anyway because if it were an Infection he'd be dead by now from it...
