You will not find what you are after easily, if at all. Your best bet on that will be second hand and even then it will take some serious rummaging. Try the cycle forum.
For new your best bet is to go to a proper bike shop with an account with someone that can get Wilkinson or maybe Target wheels. Bob Elliott’s are probably the most likely to have them. We had a lot of student bikes that were mid 60s to deal with and I don’t think we ever found any 650b wheels that were suitable for that sort of money.
Are you sure it is 650b? Not one of the myriad of other wheel sizes available then?