If you want a slow card, buy a cheap slow CF card.
You can get off-brand 32 GB CF cards for $30 - $40 at Amazon. You should be able to pick up 16 GB cards in the impulse bin in the checkout line at your local office supply store for $15 - $20. Personally, if I were to go that route, I'd get a half-dozen 8 GB cards (for $10 - $12 / ea)...if a card goes bad, you've limited the damage.
The only reason you'd want to use an adapter would be for one of those specialty cards, like the EyeFi, or if you were going to take the card out of the camera and put it directly in something that doesn't support CF (such as a cheap picture frame), or if you were going to immediately hand the un-edited JPEGs to a client who only wants SD. None of those situations are common or, frankly, even a good idea -- there're other, much better ways to accomplish the same thing. Granted, those other ways are more expensive...but if you've just blown thousands on the camera, why're you fretting over tens of dollars for this sort of thing?