I don't mean to be gruesome, but does anyone know where I can purchase a miniature coffin? (about 5" long)

Three suggestions

Now that Halloween is not that far off some stores are already putting out Halloween decorations. There are a few haunted villages out there and I would think coffins were available.

Also check out some diecast car web sites, Asheville Diecast and Diecast Direct. There are several hearses with coffins available. A 1/24 scale or 1/25 scale coffin would be about 6 inches long. The 1/18 scale hearses would have a coffin about 7-8 inches long.

You could make one, I mean its just a basic rectangular box. You could use a block of wood and glue bamboo skewers for handles. A little paint and it should look fine.

I said 3 here's a bonus. I don't like saying ebay for every answer because I think it's a cop out but look under "model trains-g scale and search for a coffin, any coffin in that scale should be about the size you want.

Contact your local coffin dealers. They have, believe it or not, miniature demo models. You may be able to purchase one from them. If not they can steer you to manufacturers for demos. I have seen coffin key chains.