Distribution methods varied worldwide. In North America, the Eon
Ticket could be transferred to the games through Mystery Events using
an e-Reader and the Eon Ticket e-Card. In Europe, it was possible to
get the Eon Ticket at promotional events, but it could also be
downloaded through Mystery Events from special Nintendo cartridges via
Game Link Cable.
A player that received their Eon Ticket from either the e-Card or a
promotional event is able to share a limited amount of Eon Tickets
with other players via record mixing. However, Eon Tickets received
via record mixing cannot be shared further. Record mixing is the only
way to obtain the Eon Ticket in Emerald.
A physical copy of the ticket was available through Nintendo Power
magazine, volume 173.
So... apparently the only way to legally get one in present time is Record mixing with someone. Unfortunately I don't know anyone who's just saving 1 ticket all these years... Finding an unused card online won't work either, as the event expired on September 1st, 2003. The only other way is through... hacking.