What are the best current ways to book a specific fare class?

I want to book D Class (for the added elite mileage on AS), but the Finnair website puts me into I Class. I called Finnair and the agent said he could not do so. Is that really the case? HUCA? I would prefer to buy direct, especially as my plans may change.

I tried BookWithMatrix and it gives me a few choices such as Priceline, which is a bit opaque in terms of details and they want to charge for any changes or refunds.

I might consider calling Amex Platinum although I have not had great results with them in the past, and they also charge, although I would rather work with them than Priceline or some other OTAs.