The only one I have found so far is this one(link below), but its a bit pricey. Have you thought about making one? You could make one from paper mache(sp?), paint and fill it with something heavy to keep it from blowing away, and coat it with some kind of sealant to keep water out. Or, your could find a nice piece of wood and try your hand at carving it out, but that sounds pretty difficult.
I would also try google.com or froogle.com and search for moai.
Hope this helps.

Here is a link that might be useful: Design Toscano

I make some garden statues and moai heads that you might be interested in. Im based in Paris Ontario Canada Moai Head Statues

check local flea markets, craigslist, pottery shops.

A guy sells them at our local farmers' market, but shipping would be expensive!