You can use the ones you found, no problem. Throw the ones with dirt in them in a bucket with some water and a little dish soap to soak out the dirt, then air dry or stick them in the oven at 200 or so till they are dry. Check for splits and cracks.
I tumble before I decap/size- keeps the flash holes from getting media in them. You can pick up either ground cob or ground walnut at Wally World in the pet section. It's a lot cheaper there than sporting goods stores, and does the same job. Walnut will clean faster but does not put as much shine on the cases as cob. I tumble with a 50/50 mix of cob and nut, seems to clean and shine pretty well. You might want to add a cap full of car wax (Nu Finish) for more shine. As you use the media, you will find your tumble times become longer to get the same results, I can't really say how long it lasts...I change it when I think it's taking too long to do the job- maybe once or twice a year.