You can trade for it at the GTS. (Global Trade Station)
by breeding two raichus
Yes, you can have one by breeding it with a male Pikachu and a female Pikachu or a Pikachu and a Ditto. If you're Pikachu has a Light Ball, maybe your Pichu can have Volt Tackle
Yes, Pichu is in SoulSilver. It originated in the Johto region. It can be found by breeding two Pikachus.
In Ilex Forest, by Event only.
By breeding a female Pikachu or Raichu with a male in the same egg group. Though if you're talking about the special Spiky Eared/Notch Eared Pichu, that event has long since passed, and you will need to trade with someone who has gotten the Pikachu Colored Pichu/Shiny Pichu event Pichu and bring it to Celebi's Forest Shrine.
You can only get pichu by breeding a pikachu (preferably with another pikachu). pikechu has a 5% appearance rate in viridian forest so just wander around over there and you will get a pikachu in approx 10 mins if you're lucky.
Pichu
Breed Pikachu with Light Ball to get Pichu with volt tackle you can also interact in Ilex Forest
no
get a pikachu from viridian forest breed it with a ditto and hatch the egg
You get it at Gamestop, but this event is already over. Sorry guys that did not get it!
Pretty sure baby Pokemon cannot be found as field Pokemon.