You can use a cheat device like an action replay and hack it or you can get it using a event. but if you don't want to cheat or wait for a event you can go on certain websites and trade with people they will trade for just about anything :)
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You can only get with action replay or an event. its hard I'm not a expert but your pose to get a ticket from wireless ports sotry doing that connect to a wireless person in the Pokemon center You can´t catch it in hg or ss, but you can trade it from a gameboy game if you have deoxys on it, to do this you need a ds lite or ds but not a dsi or dsixl as they don´t have a gameboy slot at the bottm. You cannot catch Deoxys in HGSS. Either hack or trade. deoxys is a new Pokemon you can not catch him.Hacking and migrating a (hacked) event one from emerald
migrate via ds or ds lite. then go to pal park to catch the ones you migrated
insere Sapphire gba in your ds and go to iron island 1st floor.
Catch a female wurmple Or if you have Pokemon ruby (gameboy game) then just plug that in to your DS when you play Pokemon platinum. Go to route 205 Eterna Forest and cascoon will have an increased chance of appearing.
In Pokemon Diamond and Pearl and Platinum which are the newest versions on the DS until the newer ones you catch Rotom after you get the national pokedex. Then you can transform him into different types.