the few ways to get marrow is by fighting the rathalos, do the capture the rathalos
quest(you obtain more rarer items from it,or you can do the secret offline quest
Troublesum Pair(fight both the rathalos and rathian) and after you complete that quest
there is a chance you get the marrow

Do Quest Attack of the Rathalos ....7 times u get wyvern marrow ....wyvern marrow very
rare items .....Rathian idont know because i beat rathian 7 times but dont get wyvern
marrow ....Kill ratalos using flash bomb...

You need fight rathalso in quest called catch a rathaslo and final quest like a trouble
some pair hunt for rathian and rathain.. You can get reward.

There are found in two quest like catch a rathalos and a troublesome pair. And it did no
found in attack of the rathalos quest.

I hope this help

You do better to kill rathalos with best blade.