Pre LC bikes it is a case of replacing the whole sender, on later bikes the valve assembly is separate.
On ebay there are Chinese senders which are usually good and all of the ones that I have used have come with the BMW part number.
About £25 a pair on ebay or £19 a pair from china.
It makes life easier if you can buy them with the codes but if not it is possible to retrieve the codes , loads of info on here.