Here's a few ideas I found.
There's a ton more, so give it a lookie. I was a little overwhelmed since I had only 1 day to get stuff for the party, and also since all the pictures everything looks amazing and well, fairly professional, which correlates with expensive. I mean look at that fun polka dot fabric in the first picture. But I didn't have the budget for that.
So I give you the cheap mom's version of a Monsters Party.
Everyone likes balloons and streamers. And if you look really hard you can see on the back wall there are some cut outs. I found this free printable and printed off a bunch on cardstock, cut them out and plastered them over pretty much every wall in the house. It's also what I used to top the cake with. If I had all the money in the world, I would have bought a ton of helium balloons and filled the ceiling with them, the 6 I got looked so awesome just hanging out.
I cannot wait for my new bathroom!
Monster Jigglers. The mold is one dollar in the Jello aisle at Walmart.
Monster plates from Walmart, Monster pencils and balls from the dollar section right now in Target.
I couldn't find a link to it, but I also went into the party section at Walmart and bought the Monsters decorating kit for $6.50 that had a ton of hanging items, and three large posters. You can see two of the three in the picture below.
I used this from Pinterest and cut out the circles and taped to cups.
And then I went to this website and printed off a ton of free coloring printables so all of the 11 children could go to town.
And because I am a glutton for punishment, I topped it all of with sugar infused green and purple juice. And green twizzlers cut in half in little tin pails that we had from Easter. It was a lot of fun, and when the kids came out Saturday morning after I decorated the night before, all three of them told me "it's so beautiful!".
It took quite a long time to decorate, so we probably will leave everything up for at least a week! Now Owen says he wants a Monsters party too!