whit WI-FI CONECTION
You cannot find a deoxys normally(during game play). but you can get it during an event. first you have to get a mystery gift. if you do not have the mystery gift, it is explained on the Pokemon website. there is no deoxys event currently going on.
you can't
You need a mystery gift event, which gives you a pass to go to an island where deoxys is.
need to get their events
mystery gift
the mystery gift allows you to get rare Pokemon such as manahy, deoxys, darkrai, and arceus. Two of these Pokemon have already had their turns to go. And those two are manaphy and deoxys. So I hope that you can get the other two!
There is no other way, unless you were to trade.
At any Pokemon Mart, fill out the questionare on the counter. Go to ABC mode and do "LINK TOGETHER WITH ALL". Now, you have a Mystery Gift. To get the Mystery Gift, restart your game and press Mystery Gift on your Title Screen. Make sure you have Internet Connection but anyway, once you get the Gift (The Aurora Ticket) take it to the Vermillion Harbor and that way, you go to Deoxys's Birth place where you can catch Deoxys.
Deoxys is not a Pokemon you will be able to capture in this game. As far as I know you could only get it in the GBA games through mystery gift. If you did happen to get that mystery gift, then you can transfer it through pal park.
* the old sea chart, which is the non-cheating way to get mew * the ticket to birth island: deoxys
you go to gamestop and get a mystery gift on emerald and then trade deoxys over to sapphire or in emerald go to birth island follow the triangle until deoxys lv.30 appears. Hope I hepled!
on your D's turn the time on the date and day the mystery gift was on and make sure you also have a R4C Chip and then just the game then you'll see mystery gift but this will only work if you've used mystery gift before