If you check eBay you can find a lot of sellers of AWG #24/0.5mm/0.0220inch diameter magnet wire in lengths up to a few thousand feet.
Ah, looking by AWG and feet (instead of mm and g) seems to provide some interesting options.
Another option is to find some local scraped device like a transformer and recover wire from that device. I used to get long lengths of antenna wire that way.
I think I might end up doing this.
There is a good chance that any coil you would wind for a crystal radio isn't that particular about wire size unless you want the coil to look a certain way or a larger size is easier to wind.
I am specifically after solid core + hard insulation (ie lacquer), so I can trim the insulation off one side of the coil and make a sliding contact. I am currently using standard insulated stranded wire which makes this very hard. I also have some thinner enameled wire, but once it gets to a certain thickness this also gets delicate and fiddly.