I never thought to check on it, because the Niko D40 I used to have just used a regular SD card, and the Nikon d300 I now own came out a year later.
Now, I am not really familiar with CF cards. The manual lists the compatible oneshttps://gamefaqs.gamespot.com/a/forum/6/66db769b.png
Now, I have no idea, but can I buy any CF card? I don't even really need more than 8 gigs storage, but I feel like actually finding any of these will cost an arm and a leg vs "off brand we still make CF card???" company from the modern day
A potentially cheaper option would be a CF to SD adapter, but again I don't know about compatibility.
I don't expect anybody to have specific familiarity with the Nikon D300, but maybe some general knowledge could apply