Graham, they sell hundreds of them every Wednesday morning at Carlisle mart. Black & whites fetch up to £100, but you can pick up Jerseys and Ayrshires for tenners. And I suspect that if you approached dairy farmers who breed their own Ayrshire or Jersey replacements, they would be happy to sell young bull calves to you direct from the farm - which avoids the risk of picking up infections at the mart.
BH used to buy in calves to rear on the bucket - beefy types, to be sold as stores, or females to be kept on as suckler cows. He always says that anyone taking on young calves to rear on powdered milk is advised to get them at not less than one month old. Younger than that and they are just too likely to scour or get other problems, and die. Now that we rear bought-in calves on the Jerseys, we have found that we can buy them younger and they seem to be fine.
His other tip was to feed them only PSF (Pfizer scour forumla) for the first 24 hours, then onto the milk. Again, we haven't found this necessary when they are going onto a cow.