You have to decide, is this PC just for mining or does in need to look pretty as well? I started with a milk crate setup, and my wife had a fit about the looks of it, so I had to buy a case for my miner PC.
If you go for a milk crate, you can pick them up at Lowes or Home Depot. If you don’t have a source locally, then you can get them here
If you need to go with a case for your miner, then I would recommend a couple.
You want a case with room for the cards and good airflow as well.
This case is pretty awesome for the price