It feels like I'm busting my ass trying to level up this slime to level 10 and I'm running out of monster drops to feed it. Am I giving it the wrong items to eat, or is there a shop I can be buying the high EXP stuff? I'm just grinding through days as fast as I can just to bump him up by such a small amount, then going to the next day.

the higher gold a monster drop is worth, the more exp the slime gets.

I know that piece of advice, but thank you, I appreciate covering the basics first. 

No, I used up all of my good monster drops ages ago. At this point, I'm feeding it anything I can get. Is there a shop to buy the more expensive drops, or a monster I can farm to get them?