This was an easy peasy costume, because I already had the main ingredients. The only things I had to buy were the white gloves and red bowtie, which I got from a dollar store. Of course, not everyone has a Cat in the Hat hat lying around. I have had it for years, it's just a random item I bought because I knew it would be handy one day. I tend to do that a lot. That's why I have so much stuff.
So, once you have the hat, gloves and bowtie, all you need are some black and white clothes. And if you wanted to be more authentic, you could add face paint - White face, black nose and whiskers. I also should have had a black tail, and I tried to find one, but with no luck, so I will keep an eye out for one in the future.
This is a really good time of year to find a few key items to put away for when you may need a costume, being Halloween month. Just about anywhere you go has Halloween stuff out at the moment, but as I said earlier, Halloween isn't a big thing here, so after Halloween you can go buy stuff on clearance for next to nothing. Hats or head accessories are always good to have, because you can usually put together a costume to go with a hat from normal everyday clothing items.