Hallowed Fountain

({T}: Add {W} or {U}.)
As Hallowed Fountain enters, you may pay 2 life. If you don't, it enters tapped.

Plains

{T}: Add one mana of any color that a land an opponent controls could produce.

{T}: Add {C}.
{W/U}, {T}: Add {W}{W}, {W}{U}, or {U}{U}.

{T}: Add one mana of any color in your commander's color identity.

{T}: Add one mana of any type that a land you control could produce.

{T}, Pay 1 life, Sacrifice Flooded Strand: Search your library for a Plains or Island card, put it onto the battlefield, then shuffle.

Sea of Clouds enters tapped unless you have two or more opponents.
{T}: Add {W} or {U}.