To find the column location of your three channels in the list of channels you would use ismember. You then use the second return value to index into your data cell array.
seekedchannels = {'AG5', 'AG7', 'AG19'};
seekeddata = cell(size(data));
for cidx = 1:numel(data)
[~, location] = ismember(seekedchannels, channel{cidx});
seekeddata{cidx} = data{cidx}(location);
end
cellfun would not be so convenient in this case, because you can't grab the 2nd return value of ismember with an anonymous function.
