Don't bother with primal spear, its a novelty weapon and completely sux, instead go for garuga or remobra or gravios lance for poison attribute. But u can find master's skull at high rank quest, basically in the wyvern's nesting/resting place, in the pile of bones, and also felyne's gathering area in different region, near the cart used to carry u when u r knocked down. Also u can find in the secret area which appears at a 3% rate at the start of high rank quest, u can increase the chance to 100% by having felyne kitchen skill explorer. If u already got unknown skull, then find in that place in high rank quest. All the quest given by felyne elder in village and the green lady in the guild are high rank quest.

Some people like novelty items....like me :) it's fun to collect.

I was able to find a Masters Skull from the guild hall high rank quest where you hunt the Giadrome/gathering quest in the mountains. I recall getting the skull in area 5...in that area you know where the eggs on that log are? well (standing on the logs and looking towards the southern end of the room) you'll see some white shapes under the ice. they are white. bones :) just go over there and loot it till you get the skull. I don't think you'll get it every time so you may have to farm it.

sorry for such a long answer haha. hope I helped.

Good Luck!

"white shapes under the ice. they are white."

Sorry... rhetorical(if that's even how you spell it) statment there haha.