Tony,
They're not cheap--about $50 U.S. for the Edirol UM1, which is the model I use. They're available in most music stores and from some of the online stores as well.
Gary
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)