There are a few places where guitar effects pedals can be purchased. There are many stores where a guitar effects pedal could be bought like Best Buy or Amazon.

Many big church organs have several keyboards and a whole set of pedals. Some complex harpsichords are also that way.

The piano has a few pedals for some effects.

No, they only need an amplifier. Pedals would be used for adding effects to change the sound etc.

Bass pedals are used to add sound effects to a bass guitar. Some of these sound effects include distortion, pre-amps, chorus, and more. They can be found at Guitar Center and Musician's Friend.

Guitar effects pedals can be purchased from all good musician and instrument stores. Check the local area. Alternatively, look on specific musical store webpages or Ebay.