Just do combo quests, you seem to have a higher chance of getting them in two/three monster quests.
I got one in the Drink tri-Naruga quest, I also got one in the elder Naruga/Jinouga quest.
Unfortunately, I didn't really give a flying **** after I got it, as I need other stuff, but I distinctly recall getting it from those quests. --- My hair ain't yellow, and my eyes aren't green, it's more like the other way around... Well, I got a real pretty mouth...underneath all the foaming - Nice Cave
Did the drink quest got one after cutting off all tails and capping all. Did it a second time capping one and cutting two tails but got two marrows. --- http://s224.photobucket.com/albums/dd126/rockmanx00/
Narga's marrow is a tail and capture reward. 3 Narga hot spring quest also has it as a quest completion reward.
Jinouga's plate was easier for me to get (2 plates out of 3 fights). Narga... needed 8 marrow (now going back and getting 3 more for upgrades) and fought him well over 20 times. --- I'm a house wife. I have nothing better to do. That's why I post here! GameFAQs: A mindset of less than 1% of any given game's total populace.