Maybe you could find an actual small anvil at an 'antique' store or flea market? I've got a small 30-40 lb. one that grandpa had that went to my dad on the farm that now lives in my garage so I would think some of the small ones are still around in rural areas.

I'm in Iowa and found several in the state for sale using craig's list and a general search on the web. If you can't source it locally, shipping costs would be a deal breaker! We've also got some blacksmithing groups and in the Des Moines area is the Living History Farm where there would be a bunch of folks that may know of a local source to get a small anvil.

If you can't do that or something like that - check out steel targets and with the local gun shops or shooting groups. There are several of those that are thick steel, come in several (flat) shapes and sizes that could work for a flat hard surface. Another idea is to check with any local metal shops in your area for a chunk of "scrap" they may have and/or a heavy equipment or vehicle shop could have some 'junk' parts from heavy equipment or engines you could adapt for your use. Maybe a 'junkyard' if there is still such a thing anymore.