So far you need to get one from the trade center you can't catch it.
You can't get manaphy on indigo.
the only way to get a manaphy in Pokemon indigo is to trade it publically or ask one of your indigo friends to trade it privately if they have a manaphy Hope i Helped You! :) -Marshmellow10
look up cheats to get manaphy
in the underwater zone or go to the trade center to trade for one
underwaterzone
To receive a Manaphy in Pokemon Platinum, you must send over a Manaphy Egg from the game Pokemon Ranger after completing a special mission involving Manaphy.
you don't get them together you have to find the sea crown and get manaphy from Pokemon ranger
you get the manaphy egg in Pokemon rangers and send it into Pokemon diamond or pearl
You can not find or get manaphy on Pokemon diamond but you can by hacking, trading or... if you have Pokemon ranger you have to complete the manaphy mission and then you can put it onto diamond.
To get a Manaphy in Pokemon Diamond you have to get Pokemon ranger and defeat it and you have to trade it to Pokemon Diamond,Platinum or Pearl.
There is no trainer in Pokemon Platinum with a Manaphy. However, you can trade with someone who has a Manaphy, or complete the special Ranger mission in Pokemon Ranger and transfer the Manaphy egg over to Pokemon Diamond, Pearl, or Platinum.
Entei can be found in Nimbasa City in Pokemon Indigo.