there are a number of possible causes of respiratory infections - it could be an allergy, fungal infection, bacterial or viral.
IMO treating respiratory diseases without a confirmed diagnosis is not ideal, but personally vets are not an option. You may wish to consider the following:
The bedding that you use may be causing an allergic reaction
The rains that you have experienced may have encouraged fungal issues
Maybe try changing the bedding first, as thats the easiest route. I'm honestly not sure about trying to treat fungal infections (maybe type that in the search box and see what pops up).
In the meantime, adding vitamin supplements / electrolytes to their water will help boost their immune system and thus help them to overcome whatever the problem may be (if its bacterial or viral).
Antibiotics will only help fight off bacterial problems, but can help in preventing secondary infections if the route cause is viral. If changing the bedding does not work, i would personally treat with vitamins / electrolytes and antibiotics (its important for me to mention that not all members advocate this route of treatment).
Some respiratory infections are only as harmful as humans getting a cold, others a lot more serious. Assuming your flock survive and get over the illness, bear in mind that they may be carriers of the illness for life and will pass it to new flock members. It would be unwise to sell or give any of your flock to other chicken keepers in the future.
I'd suggest further reading on the subject as it can be complicated.
All the best