Haven't used one myself, but I would check https://www.gundogsupply.com to see what the options are. I've ordered other things through them and had a good experience.
Just guessing from your description, it would really depend on what caused it. If a true pressure sore then I'm thinking you would want something padded, but if it's more of a scrape or puncture then probably either neoprene (although that might get hot) or stiffer. Not sure the brush guard ones would be enough, although it might depend on what they were made of. Not sure you'd want a stiff plate on a pressure would as it might just aggravate the pressure.