There are a few dealers like Harris Cyclery that stock old English bike parts, but your best bet is eBay. The BSA bicycle division was bought out by Raleigh in 1957. Sometime soon afterwards, bikes labeled BSA were actually Nottingham built Raleighs and many of the parts aren't interchangeable with the older Birmingham built bikes. The hub date suggests that it's a Raleigh built bike, but it's best to check other details to be sure. Wheels can be changed. Sturmey Archer parts are usually good across the board, still there's minor changes and cable length differences to watch out for.