Thanks for your interest! The vest was originally a sleeveless button-up shirt that I just shortened and removed the buttons from. The style of the shirt was one that had the button holes covered by another strip of fabric, so they're there, just not visible from the outside. The boots I bought as-is from the clothing store Rainbow. They tend to sell cheap, colorful boots. I got lucky that that year they were selling ones in just the right color!
I generally have better luck looking for specific color/styles of shirts at thrift stores (which is where I found this one) than regular stores. As for the boots, I haven't seen this style or color at Rainbow in a few years, but inexpensive junior's clothing stores tend to have similar types of shoes.
I think it's just a cotton/synthetic blend, medium weight, but a little stiff. Basically, besides color, I looked for one that was fitted (had some shape to the way it was sewn rather than just being a boxy-shape) and fit snugly since it needed to stay open. I think I also took the sides in a bit to make in smaller so it wouldn't close, but if you can just find one a little too small, that would probably work too.