Just search etsy for running costume. There are makers who specialize in Disney and will do custom orders. You can also Disney Bound with regular running clothes. There are a couple of companies who do Disney themed running shirts, leggings, shorts and skirts.
It also can't hurt to message sellers if you have a specific question. One seller has Disney Princess Mandalorian armor shirts, but listed in women's sizing on their page. As a man running the 2021 princess races virtually, I wanted to get in on the fun so to speak and I messaged the seller to see if I could get the shirts I wanted in unisex sizes. The seller said that was possible and did a custom order for me.
Yes, lots on Etsy. So many, that it's daunting. Any specific vendors that are recommended so that I can narrow my searches?
And Re Disney Bounding -- Yes I think I will need to wear reliably comfortable regular running clothes for the longer races, so this is a great suggestion!
A couple of timing related issues to remember here. First off, it's a good idea to do a training run in race clothing, especially costumes. You will want to know about any possible serious problems before the race itself. You might have the best race costume ever in pictures, but it might be very uncomfortable and/or cause issues such as chafing on race day itself. Race costumes can be achieved with multiple layers, but you will want to know how these layers interact with your body during a run.
I've run some races with a Yoda backpack recreating the scene from The Empire Strikes Back. Sometimes as Luke Skywalker, once as Batman, and once in a normal running shirt. The backpack can slip off your shoulders when you run, so you may be constantly adjusting it. May not be an issue for a 5K, but you may feel differently constantly adjusting it during a Half Marathon.
Furthermore, some race clothing sits differently on your body than others. Not all moisture wicking fabrics are created equal. If the costume is for a longer distance, I recommend trying to a longerish run in that costume. I've had race shirts that literally stick to my body. I can deal with that for shorter distances, but for Half Marathon or more, I want the shirts that have consistently proven to work in training and previous races.
So when ordering from etsy, pay attention to delivery time. Will it arrive before your race? Will it arrive in time for you to test it out on a run (or maybe multiple runs) before your race?