The term Finisher comes from Commercial Poultry operations, it is designed to put weight on birds quickly in the last couple of weeks before processing. You can;t always find it in Farms stores where most of use get our feed from. But it is usually a custom mix that some feed store have bagged or can make if they have there own feed mill. The commercial growers in our area usually make there own. It's usually used in the last couple of weeks before processing.
But I have been told that if you mix a 22 % or so wild game bird feed with cracked corn at 50% for each by volume, some times about 5 to 15% sweet feed is added but not always, is the same thing. But for most people with small flocks just using what every you used as a grower ration will work.
Sweet feed is a general purpose bagged animal feed with molasses mixed in to coat the feed.
As small flock owners we are not always trying to duplicate what is done in a commercial operations. So using a special finisher is not always the best way to go or even practical for most people. My wife switched our last batch to a finisher and I noticed they did have some extra fat on them. But they were red sex link males which is rather slim bird to begin, and I haven't butchered them before, so I can't say for sure how well it works for small flocks. Some of our other heavey breed broilers do appear to be a bit larger then what I remember them to be but I didn't weight or measure them.
Tom