Every year, I make all my grandchildren's costumes. I use plain sweats in bright colors without any logo's on them. Hanes is a good one to use. I have fabric left over from past Halloween crafts that I use. Once the heat bond cloth is in place, I iron pieces to the sweat shirt or pants, then use fabric paints to personalize them.
I use glow-in-the-dark paints so they can be seen at night. I make them as bright and glittery as possible, as the kids seem to love them that way. Once they are worn at Halloween, the kids use them for Winter pajamas.
I make them and send them a month in advance, so they can enjoy the costume before Halloween.