I have never managed to get a refill at an airport anywhere outside the UK; certainly not a 540 thread. A firm at Biggin once did me for £75...
The bigger airports (not ones in the 3rd world) tend to have oxygen servicing for bizjets but these use an unusual adaptor; nothing like a 540.
I carry a 540 to standard BOC bullnose adaptor and using that once managed to get a refill in Crete, which took an all-day taxi ride around Hania until I ended up in an industrial gas factory (recommended by the uncle of the brother of somebody working in a scuba shop).
Some scuba shops do o2 so you could try those, but you will most likely need the bullnose adaptor. I have found UK scuba shops hopeless; even those which carry o2 soon suss out you are not a diver and get "funny" about it.
Sorry for the likely bad news. I gave up on this lousy time wasting business 2 years ago and bought a 48 cu ft carbon/kevlar 540-thread bottle (about $600) from Mountain High in the USA, which does the longest possible trip, and I refill it from a welding o2 cylinder which I rent from BOC (£80/year, £20 for a swapout) and keep at home
Alongside an argon bottle I use for TIG welding but I don't breathe that stuff; not least because argon costs a whole lot more than o2
Email me if you need details - I have all this written up.