I know the receiver is part of the aerial, the statement seemed to put across that it was a seperate entity.
They are pictured on seamark nunn at £99.
I would also be prepared to be disapointed with the sounder, in fact I would be go as far as to recommend that you don't fit it and either return it or put it on ebay as they are useless. I fitted a 240 blue two months ago and am now seriously considering taking it off and selling it to fit a lowrance or similiar (it is that bad)
I have the 182 plotter and think it is fantastic, don't know why the sounders are so cr**