These business cards holder sheets worked great and they were cheap:
Business card holder on Amazon
Basically you can fit 3 medium sized pawns in one business card slot and one large sized monster will fit in sideways. I know a few people were asking for good ways to store these and I think this works great. I keep the PCs separate and have the monsters alphabetized to easily find them. I imagine any business card holder sheet would work, but I have only tried the one linked.
I have all my Beginner Box pawns in 6 sheets, but if I wanted to I could probably get it down to 3 or 4 sheets by combining monsters in the same business card sleeve. I like to keep my monsters separated for whatever reason.