That appears to be an output transistor (ie one end goes out to the speaker terminal) There are typically multiple of these running in parallell, as a single one couldn't handle the current required and/or the power dissipation (heat losses).

A google gives a lot of sources, for instance www.mouser.com

Edit:
Could also be used for other purposes, but if it was attached to the heat sinks then it's most likely used as an output transistor.

In Germany I think Conrad would be a place that would have them.

/Michael