I have to agree with Castle Farm, you all too often see 6-18month old collies advertised as working dogs, and if they were really working that well, they wouldn't be advertised for sale. You are best to find someone nearby who has working collies who herd sheep, and get one from them. Our 2 year old has a fantastic instinct (we don't work him as we have no sheep, and goats don't like to be herded much!) but he came from a local farm, where the farmer has his mother, father, brother, and now a new little sister, which are all working sheepdogs.
If you are fortunate, you could find someone who would let your dog do a bit practice of on their sheep with his parents etc.
Beth