honestly, if you can't get a railroad set up, I wouldn't rule out air transport so quickly. the cargo hold where animals travel is climate controlled with an absolute maximum temperature of 80F (which is the highest end of the "safe" range for bunnies). I'd make sure to get them on a direct flight, though - avoid any itineraries with layovers/flight changes, as that's where problems tend to arise (I have a friend who purchased gliders that had to be shipped... it wasn't a direct flight and they ended up getting stuck somewhere for 24h+. they were cared for by an animal boarding facility in the meantime and made it to her safe, sound and healthy in the end but it was definitely nerve-wracking).
the majority of sugar glider breeders ship - in the glider world, sticking suggies in an airplane's cargo hold is very much common practice. if someone's selling "pet only" standard grays, they can probably do most or all of their business locally, as people who are just buying lower priced gliders as pets don't typically want to shell out $200 for shipping on $200-300 worth of gliders.... however, if you sell "breeding stock", breeders are more than willing to pay shipping fees to get gliders with the right lineage to be compatible with their breeding program if they can't find anything locally. gliders get shipped year-round, even in the summer, without issue. all airlines have cut-off points where the outside temps are too hot or cold (at one or more points on the trip) for them to guarantee that they'll be able to keep pets at an appropriate temperature at all times and they'll refuse to accept animals on cargo flights on those days.
imo, it's at least worth calling airlines and asking them about the temperature ranges bunnies might experience at various stages of travel - chances are, you can find one that gives you satisfactory answers
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